apply_condition: notequal need not have constant operand

Just as is done for equality, the not-equal case needs to test that the
LHS is a symbol and the RHS is a constant before attempting to constant
propagate.

The problem became apparent while trying to enable abort-after-failing-C
assert, but only on Visual Studio builds (owing to the code that the
assert macro expands to in Visual Studio).

Author: tautschnig

src/goto-symex/goto_state.cpp

@@ -141,6 +141,9 @@ void goto_statet::apply_condition(
     if(is_ssa_expr(rhs))
       std::swap(lhs, rhs);
 
+    if(!is_ssa_expr(lhs) || !goto_symex_is_constantt()(rhs))
+      return;
+
     if(rhs.is_true())
       apply_condition(equal_exprt{lhs, false_exprt{}}, previous_state, ns);
     else if(rhs.is_false())