Don't warn on array_of_expr with zero length

These result from array_set instructions, which when targeting
variable-length arrays can end up targeting zero-sized arrays.

Author: smowton

src/solvers/flattening/boolbv_array_of.cpp

@@ -25,7 +25,7 @@ bvt boolbvt::convert_array_of(const array_of_exprt &expr)
   std::size_t width=boolbv_width(array_type);
 
   if(width==0)
-    return conversion_failed(expr);
+    return bvt();
 
   const exprt &array_size=array_type.size();