maint: fix operator precedence in mbrtowc test This is a fix for test breakage introduced by commit 45228d96; the equality expression must be parenthesized when negated with '!', otherwise we always get: test-mbrtowc.c:49: assertion 'ret == (size_t)(-2)' failed * m4/mbrtowc.m4 (gl_MBRTOWC_EMPTY_INPUT): Negate the entire expression. * m4/mbrlen.m4 (gl_MBRLEN_EMPTY_INPUT): Likewise.