compat_urllib_parse_unquote: crash fix: only decode valid hex

on python 2 the following has a { "crash_rate": "100%" } of the time
as it tries to parse '" ' as hex.

youtube_dl/compat.py

@@ -94,6 +94,8 @@ except ImportError:
             try:
                 if not item:
                     raise ValueError
+                if not re.match('[0-9a-fA-F][0-9a-fA-F]',item[:2]):
+                    raise ValueError
                 pct_sequence += item[:2].decode('hex')
                 rest = item[2:]
                 if not rest: