Fix path length overflow in realpath [BZ #22786]

Integer addition overflow may cause stack buffer overflow
when realpath() input length is close to SSIZE_MAX.

2018-05-09  Paul Pluzhnikov  <ppluzhnikov@google.com>

	[BZ #22786]
	* stdlib/canonicalize.c (__realpath): Fix overflow in path length
	computation.
	* stdlib/Makefile (test-bz22786): New test.
	* stdlib/test-bz22786.c: New test.

(cherry picked from commit 5460617d)