Remove try! statement from FailableIterator

My team has recently experienced crashes of an app in production related to SQLite.swift. Upon further review, we discovered that these crashes are occurring due to this `try!` encountering a `throw` from `failableNext()`.

This PR resolves the issue by returning nil if a throw is encountered.

Here is the entire thread that crashes. Note that it occurs when using the Apollo-iOS framework, found (https://github.com/apollographql/apollo-ios)[here]:

```
Thread 7 name:
Thread 7 Crashed:
0   libswiftCore.dylib            	0x000000010117f968 0x100fa0000 + 1964392
1   libswiftCore.dylib            	0x000000010117f968 0x100fa0000 + 1964392
2   libswiftCore.dylib            	0x000000010102f994 0x100fa0000 + 588180
3   SQLite                        	0x0000000100d7e80c _T06SQLite16FailableIteratorPAAE4next7ElementQzSgyFAA9StatementC_Tg5 + 212 (Statement.swift:211)
4   SQLite                        	0x0000000100d674c8 _T06SQLite10ConnectionC7prepares11AnySequenceVyAA3RowVGAA9QueryType_pKFs0D8IteratorVyAHGycfU_AHSgycfU_TA + 76 (Query.swift:927)
5   SQLite                        	0x0000000100d67578 _T06SQLite3RowVSgIxo_ADIxr_TRTA + 48 (Query.swift:0)
6   libswiftCore.dylib            	0x000000010114e9bc 0x100fa0000 + 1763772
7   libswiftCore.dylib            	0x000000010114e9fc 0x100fa0000 + 1763836
8   libswiftCore.dylib            	0x000000010114ead0 0x100fa0000 + 1764048
9   libswiftCore.dylib            	0x000000010102a8c8 0x100fa0000 + 567496
10  libswiftCore.dylib            	0x000000010114e16c 0x100fa0000 + 1761644
11  libswiftCore.dylib            	0x000000010118fd78 0x100fa0000 + 2030968
12  libswiftCore.dylib            	0x0000000100fa8774 0x100fa0000 + 34676
13  libswiftCore.dylib            	0x000000010115f5bc 0x100fa0000 + 1832380
14  libswiftCore.dylib            	0x000000010114f4fc 0x100fa0000 + 1766652
15  Apollo                        	0x0000000100a01c3c _T06Apollo21SQLiteNormalizedCacheC13selectRecords33_0382DE2BE0B0836588D3F2CA65C602E7LLSayAA6RecordVGSaySSG7forKeys_tKFTf4gn_n + 1284 (SQLiteNormalizedCache.swift:0)
16  Apollo                        	0x0000000100a02710 _T06Apollo21SQLiteNormalizedCacheC12mergeRecords33_0382DE2BE0B0836588D3F2CA65C602E7LLs3SetVySSGAA06RecordO0V7records_tKFTf4gn_n + 116 (SQLiteNormalizedCache.swift:0)
17  Apollo                        	0x0000000100a031c8 _T06Apollo21SQLiteNormalizedCacheC5mergeAA7PromiseCys3SetVySSGGAA06RecordG0V7records_tFTf4gn_n + 304 (SQLiteNormalizedCache.swift:0)
18  Apollo                        	0x00000001009ff290 _T06Apollo21SQLiteNormalizedCacheCAA0cD0A2aDP5mergeAA7PromiseCys3SetVySSGGAA06RecordG0V7records_tFTW + 44 (SQLiteNormalizedCache.swift:0)
19  Apollo                        	0x000000010099d1ec _T06Apollo0A5StoreC7publishAA7PromiseCyytGAA9RecordSetV7records_SvSg7contexttFyyyc_ys5Error_pctcfU_yycfU_Tf4ggng_n + 164 (ApolloStore.swift:52)
20  Apollo                        	0x000000010099f788 _T06Apollo0A5StoreC7publishAA7PromiseCyytGAA9RecordSetV7records_SvSg7contexttFyyyc_ys5Error_pctcfU_yycfU_TA + 104 (ApolloStore.swift:0)
21  Apollo                        	0x00000001009c0f1c _T0Ix_IyB_TR + 36 (GraphQLQueryWatcher.swift:0)
22  libdispatch.dylib             	0x00000001812b2a54 _dispatch_call_block_and_release + 24 (init.c:994)
23  libdispatch.dylib             	0x00000001812b2a14 _dispatch_client_callout + 16 (object.m:502)
24  libdispatch.dylib             	0x00000001812c1540 _dispatch_queue_concurrent_drain + 540 (inline_internal.h:2500)
25  libdispatch.dylib             	0x00000001812bd304 _dispatch_queue_invoke$VARIANT$mp + 348 (queue.c:5304)
26  libdispatch.dylib             	0x00000001812bfcf4 _dispatch_root_queue_drain + 600 (inline_internal.h:2539)
27  libdispatch.dylib             	0x00000001812bfa38 _dispatch_worker_thread3 + 120 (queue.c:6104)
28  libsystem_pthread.dylib       	0x000000018155b06c _pthread_wqthread + 1268 (pthread.c:2286)
29  libsystem_pthread.dylib       	0x000000018155ab6c start_wqthread + 4
```

Author: ethansinjin

Sources/SQLite/Core/Statement.swift

@@ -208,7 +208,10 @@ public protocol FailableIterator : IteratorProtocol {
 
 extension FailableIterator {
     public func next() -> Element? {
-        return try! failableNext()
+        if let next = try? failableNext() {
+            return next
+        }
+        return nil
     }
 }