Fix a bug that error in expression may cause stack overflow

If stack overflow is caused, groonga crashes. The added test is
crashed without this fix.

The fix clears ctx->impl->stack_curr even if an error is occurred in
the execution. We need more work to make groonga more robust. We
should check return value of grn_ctx_push() everywhere such as
grn_normalizer_normalize(). Without the check, groonga will crash.