fnmatch(3): improve POSIX conformance.

In a recent interpretation[1], "\\" shall return a non-zero value
(indicating either no match or an error).

The fix involves a change over r254091 and now the behavior matches the
Sun/IBM/HP closed source implementations and also likely musl libc.

Submitted by:	Joerg Schilling <joerg at schily.net>
MFC after:	1 week

[1] http://austingroupbugs.net/view.php?id=806

lib/libc/gen/fnmatch.c

@@ -184,7 +184,8 @@ fnmatch1(const char *pattern, const char *string, const char *stringstart,
 			if (!(flags & FNM_NOESCAPE)) {
 				pclen = mbrtowc(&pc, pattern, MB_LEN_MAX,
 				    &patmbs);
-				if (pclen == (size_t)-1 || pclen == (size_t)-2)
+				if (pclen == 0 || pclen == (size_t)-1 ||
+				    pclen == (size_t)-2)
 					return (FNM_NOMATCH);
 				pattern += pclen;
 			}