Narrowing the scope of the global extern phi merge check

The is_not_nil check was a little broad and causing uninitialized values to get merged in some situations. Made the condition for being an external global stricter.

Author: JohnDumbell

src/goto-symex/symex_goto.cpp

@@ -476,15 +476,21 @@ static void merge_names(
   //  1. Either guard is false, so we can't follow that branch.
   //  2. Either identifier is of generation zero, and so hasn't been
   //     initialized and therefore an invalid target.
+
+  // These rules don't apply to global externs, which can be uninitialized and
+  // are still valid targets.
+  bool is_global_extern =
+    symbol.is_extern && symbol.is_static_lifetime && !symbol.value.is_not_nil();
+
   if(dest_state.guard.is_false())
     rhs = goto_state_rhs;
   else if(goto_state.guard.is_false())
     rhs = dest_state_rhs;
-  else if(goto_count == 0 && symbol.value.is_not_nil())
+  else if(goto_count == 0 && !is_global_extern)
   {
     rhs = dest_state_rhs;
   }
-  else if(dest_count == 0 && symbol.value.is_not_nil())
+  else if(dest_count == 0 && !is_global_extern)
   {
     rhs = goto_state_rhs;
   }