Update tst_mbrlen/tst_mbrtowc for mblen change

commit 9781a370 changed the expected
results for mbrlen in case of passing n=0 to -2. The initialization of
tst_mbrlen_loc and tst_mbrtowc should be updated accordingly.

	* tests-mbwc/dat_mbrlen.c (tst_mbrlen_loc): Change expected
	result to -2 in case of n == 0.
	* tests-mbwc/tst_mbrtowc.c (tst_mbrtowc): Check result against
	-2 instead of 0.