rtpdec: Initialize some variables to silence compiler warnings

The warnings are false positives, older gcc versions (such as 4.5)
think the variables can be used uninitialized while they in
practice can't, while newer (4.6) gets it right.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpdec.c

@@ -424,7 +424,7 @@ int ff_rtp_send_rtcp_feedback(RTPDemuxContext *s, URLContext *fd,
     AVIOContext *pb;
     uint8_t *buf;
     int64_t now;
-    uint16_t first_missing, missing_mask;
+    uint16_t first_missing = 0, missing_mask = 0;
 
     if (!fd && !avio)
         return -1;

libavformat/rtpdec_vp8.c

@@ -82,7 +82,7 @@ static int vp8_handle_packet(AVFormatContext *ctx, PayloadContext *vp8,
         keyidx_present = 0;
     int pictureid = -1, pictureid_mask = 0;
     int returned_old_frame = 0;
-    uint32_t old_timestamp;
+    uint32_t old_timestamp = 0;
 
     if (!buf) {
         if (vp8->data) {