parser: Check for eval error before memory reference parsing

If there is an error in evaluate() happened then @value
is NULL so that we better fail with error report instead
of trying processing it.

http://bugzilla.nasm.us/show_bug.cgi?id=3392290

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

parser.c

@@ -900,6 +900,8 @@ is_expression:
             value = evaluate(stdscan, NULL, &tokval, &op->opflags,
                              critical, nasm_error, &hints);
             i = tokval.t_type;
+            if (!value)
+                goto fail;
 
             if (parse_mref(&o2, value))
                 goto fail;