gdb_test_multiple: return -1 on internal error

gdb_test_multiple: return -1 on internal error

gdb_test_multiple is supposed to return -1 on internal error:

  # Returns:
  #    1 if the test failed, according to a built-in failure pattern
  #    0 if only user-supplied patterns matched
  #   -1 if there was an internal error.

But alas, that's broken, it returns success...  It looks like the code
is assuming an earlier 'set result -1' is still in effect, but
'result' is set to 0 at the end, just before we call gdb_expect:

    set result 0
    set code [catch {gdb_expect $code} string]

gdb/testsuite/
2015-03-02  Pedro Alves  <palves@redhat.com>

	* lib/gdb.exp (gdb_test_multiple) <internal error>: Set result to
	-1.