Fix a segfault when bsdgrep -i is given an empty pattern string.

PR:		bin/172865
Reviewed by:	gabor
Approved by:	emaste (co-mentor)
MFC after:	1 week

usr.bin/grep/regex/tre-fastmatch.c

@@ -468,7 +468,7 @@ static int	fastcmp(const fastmatch_t *fg, const void *data,
   fg->nosub = (cflags & REG_NOSUB);					\
 									\
   /* Cannot handle REG_ICASE with MB string */				\
-  if (fg->icase && (TRE_MB_CUR_MAX > 1))				\
+  if (fg->icase && (TRE_MB_CUR_MAX > 1) && n > 0)			\
     {									\
       DPRINT(("Cannot use fast matcher for MBS with REG_ICASE\n"));	\
       return REG_BADPAT;						\