goto-program interpreter: test and fix 'r' command

Fix basic execution as sparse vectors do not (yet) support resizing to
smaller sizes (performing a clear+resize instead).

Author: tautschnig

regression/goto-interpreter/step/main.c

@@ -0,0 +1,6 @@
+int main()
+{
+  int x;
+  __CPROVER_assert(x == 42, "should fail");
+  return 0;
+}

regression/goto-interpreter/step/r.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+r,q
+^Starting interpreter$
+^\d+- Program End\.$
+^EXIT=0$
+^SIGNAL=0$
+--

src/goto-programs/interpreter.cpp

@@ -858,6 +858,7 @@ void interpretert::execute_function_call()
 void interpretert::build_memory_map()
 {
   // put in a dummy for NULL
+  memory.clear();
   memory.resize(1);
   inverse_memory_map[0] = {};
 

src/util/sparse_vector.h

@@ -51,6 +51,12 @@ template<class T> class sparse_vectort
     _size=new_size;
   }
 
+  void clear()
+  {
+    underlying.clear();
+    _size = 0;
+  }
+
   typedef typename underlyingt::iterator iteratort;
   typedef typename underlyingt::const_iterator const_iteratort;