quotedString should verify a string is in fact quoted

This is a fairly minor issue, but can complicate troubleshooting when a user makes a non-obvious mistake in their HLS manifest.

If a user has an m3u8 like this

#EXTM3U
#EXT-X-VERSION:3
#EXT-X-MEDIA-SEQUENCE:0
#EXT-X-ALLOW-CACHE:YES
#EXT-X-KEY:METHOD=AES-128,URI=small.mp4.key

(note the lack of quotes on the value of URI) then the request VLC makes to their webserver/filesystem will be for mall.mp4.ke

This is because (in 3.0.8) the URI has quotedString() called on it in modules/demux/hls/playlist/Parser.cpp on line 286.

quotedString (defined in modules/demux/hls/playlist/Tags.cpp on line 119) doesn't check for the presence of quotes and just drops the first and last char

std::istringstream is(value.substr(1, value.length() - 2));

Although the HLS spec requires that URI be quoted, older versions of VLC handled this more gracefully.

Weatherwax (2.2.2) had an explicit check for quotes (in modules/stream_filter/httplive.c on line 836) and removed only if present

/* Url is put between quotes, remove them */
if (*value == '"')
{
    /* We need to strip the "" from the attribute value */
    uri = value + 1;
    char* end = strchr(uri, '"');
    if (end != NULL)
        *end = 0;
}

Although it only affects non-compliant streams, it seems quotedString() probably should verify that the string it's been passed is in fact quoted.