Fix environment traversal when an envvar value is empty

Fix environment traversal when an envvar value is empty

The condition when the value of an envvar is empty (not just '\0'),
the loop in tunables_init gets stuck infinitely because envp is not
incremented.  Fix that by always incrementing envp in the loop.

Added test case (tst-empty-env.c) verifies the fix when the source is
configured with --enable-hardcoded-path-in-tests, thanks Josh Stone for
providing the test case.  Verified on x86_64.

	* elf/dl-tunables (get_next_env): Always advance envp.
	* stdlib/tst-empty-env.c: New test case.
	* stdlib/Makefile (tests): Use it.