Fix signed integer overflow in random_r (bug 17343).

Bug 17343 reports that stdlib/random_r.c has code with undefined
behavior because of signed integer overflow on int32_t.  This patch
changes the code so that the possibly overflowing computations use
unsigned arithmetic instead.

Note that the bug report refers to "Most code" in that file.  The
places changed in this patch are the only ones I found where I think
such overflow can occur.

Tested for x86_64 and x86.

	[BZ #17343]
	* stdlib/random_r.c (__random_r): Use unsigned arithmetic for
	possibly overflowing computations.

(cherry picked from commit 8a07b0c4)