regex: Fix regex literal crash

Signed-off-by: Hiroshi Hatake <hiroshi@chronosphere.io>

Author: cosmo0920

src/flb_regex.c

@@ -129,7 +129,7 @@ static int str_to_regex(const char *pattern, OnigRegex *reg)
 
     option = check_option(start, end, &new_end);
 
-    if (pattern[0] == '/' && pattern[len - 1] == '/') {
+    if (len > 1 && pattern[0] == '/' && pattern[len - 1] == '/') {
         start++;
         end--;
     }