The last argument to strncasecmp is incorrect, so it matched even when
can%' wasn't followed by t.  Also, the !ISALPHA (format_chars[1]) test
looks pointless, format_chars[1] must be ' if strncasecmp succeeded and
so will never be ISALPHA.

2020-02-10  Jakub Jelinek  <jakub@redhat.com>

	PR other/93641
	* c-format.c (check_plain): Fix up last argument of strncasecmp.
	Remove useless extra test.

	* gcc.dg/format/gcc_diag-11.c (test_cdiag_bad_words): Add two further
	tests.