grep: use xsnprintf to format failure message

This looks at first glance like the sprintf can overflow our
buffer, but it's actually fine; the p->origin string is
something constant and small, like "command line" or "-e
option".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -306,9 +306,9 @@ static NORETURN void compile_regexp_failed(const struct grep_pat *p,
 	char where[1024];
 
 	if (p->no)
-		sprintf(where, "In '%s' at %d, ", p->origin, p->no);
+		xsnprintf(where, sizeof(where), "In '%s' at %d, ", p->origin, p->no);
 	else if (p->origin)
-		sprintf(where, "%s, ", p->origin);
+		xsnprintf(where, sizeof(where), "%s, ", p->origin);
 	else
 		where[0] = 0;