The first problem happens because we don't check for missing labels when parsing
'goto' statements. I.e.:

__GIMPLE() void fn1() {
  if (1)
    goto
}

The fix is pretty obvious: just add a check.
My question is: which functions should I use to produce diagnostics? The
surrounding code uses 'c_parser_error', but this function does not handle
locations very well (in fact, it uses input_location).

-- 
Regards,
   Mikhail Maltsev

gcc/testsuite/ChangeLog:

2017-05-01  Mikhail Maltsev  <maltsevm@gmail.com>

	* gcc.dg/gimplefe-error-4.c: New test.
	* gcc.dg/gimplefe-error-5.c: New test.


gcc/c/ChangeLog:

2017-05-01  Mikhail Maltsev  <maltsevm@gmail.com>

	* gimple-parser.c (c_parser_gimple_if_stmt): Check for empty labels.