mpjpeg: Cope with multipart lacking the initial CRLF

Some server in the wild do not put the boundary at a newline
as rfc1347 7.2.1 states.
Cope with that by reading a line and if it is not empty reading
a second one.

Reported-By: bitingsock

libavformat/mpjpegdec.c

@@ -153,10 +153,20 @@ static int parse_multipart_header(AVFormatContext *s)
     int found_content_type = 0;
     int ret, size = -1;
 
+    // get the CRLF as empty string
     ret = get_line(s->pb, line, sizeof(line));
     if (ret < 0)
         return ret;
 
+    /* some implementation do not provide the required
+     * initial CRLF (see rfc1341 7.2.1)
+     */
+    if (!line[0]) {
+        ret = get_line(s->pb, line, sizeof(line));
+        if (ret < 0)
+            return ret;
+    }
+
     if (strncmp(line, "--", 2))
         return AVERROR_INVALIDDATA;
 
@@ -208,9 +218,6 @@ static int mpjpeg_read_packet(AVFormatContext *s, AVPacket *pkt)
     if (ret < 0)
         return ret;
 
-    // trailing empty line
-    avio_skip(s->pb, 2);
-
     return 0;
 }