unescape: Try to plug fuzzer error

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

src/flb_unescape.c

@@ -223,6 +223,10 @@ int flb_unescape_string_utf8(const char *in_buf, int sz, char *out_buf)
                 size = end - next;
                 if (size > 0) {
                     esc_in = u8_read_escape_sequence(next, size, &ch) + 1;
+                    if (esc_in == -1) {
+                        /* skip invalid sequence */
+                        continue;
+                    }
                 }
                 else {
                     /* because char is unsigned char by default on arm, so we need to do a explicit conversion */