Fix strstr bug with huge needles (bug 23637)

The generic strstr in GLIBC 2.28 fails to match huge needles.  The optimized
AVAILABLE macro reads ahead a large fixed amount to reduce the overhead of
repeatedly checking for the end of the string.  However if the needle length
is larger than this, two_way_long_needle may confuse this as meaning the end
of the string and return NULL.  This is fixed by adding the needle length to
the amount to read ahead.

	[BZ #23637]
	* string/test-strstr.c (pr23637): New function.
	(test_main): Add tests with longer needles.
	* string/strcasestr.c (AVAILABLE): Fix readahead distance.
	* string/strstr.c (AVAILABLE): Likewise.