json-email: fix coverity alert

The code was not correct and coverity did detect a potential
overflow problem that should not happen because of the structure
of md5 string and of format.

src/output-json-email-common.c

@@ -140,7 +140,7 @@ static void JsonEmailLogJSONMd5(OutputJsonEmailCtx *email_ctx, json_t *js, SMTPT
         field = MimeDecFindField(entity, "subject");
         if (field != NULL) {
             unsigned char md5[MD5_LENGTH];
-            char smd5[2 * MD5_LENGTH + 1];
+            char smd5[256];
             char *value = BytesToString((uint8_t *)field->value , field->value_len);
             if (value) {
                 size_t i,x;