Remove return statements following infinite loops without break

These statements cannot be reached and are thus not needed.
This removes a number of compiler warnings.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/h264.c

@@ -3552,7 +3552,6 @@ static int decode_slice(struct AVCodecContext *avctx, void *arg){
         ff_draw_horiz_band(s, 16*s->mb_y, 16);
     }
 #endif
-    return -1; //not reached
 }
 
 /**

libavcodec/mpeg12.c

@@ -1928,8 +1928,6 @@ static int slice_decode_thread(AVCodecContext *c, void *arg){
         if(mb_y < 0 || mb_y >= s->end_mb_y)
             return -1;
     }
-
-    return 0; //not reached
 }
 
 /**

libavformat/asfdec.c

@@ -1099,8 +1099,6 @@ static int asf_read_packet(AVFormatContext *s, AVPacket *pkt)
             assert(asf->packet_size_left < FRAME_HEADER_SIZE || asf->packet_segments < 1);
         asf->packet_time_start = 0;
     }
-
-    return 0;
 }
 
 // Added to support seeking after packets have been read

libavformat/bethsoftvid.c

@@ -220,8 +220,6 @@ static int vid_read_packet(AVFormatContext *s,
             av_log(s, AV_LOG_ERROR, "unknown block (character = %c, decimal = %d, hex = %x)!!!\n",
                    block_type, block_type, block_type); return -1;
     }
-
-    return 0;
 }
 
 AVInputFormat ff_bethsoftvid_demuxer = {

libavformat/mm.c

@@ -184,8 +184,6 @@ static int read_packet(AVFormatContext *s,
             avio_skip(pb, length);
         }
     }
-
-    return 0;
 }
 
 AVInputFormat ff_mm_demuxer = {

libavformat/mmsh.c

@@ -208,7 +208,6 @@ static int get_http_header_data(MMSHContext *mmsh)
             }
         }
     }
-    return 0;
 }
 
 static int mmsh_open(URLContext *h, const char *uri, int flags)

libavformat/mov.c

@@ -2308,7 +2308,6 @@ static int mov_probe(AVProbeData *p)
             return score;
         }
     }
-    return score;
 }
 
 // must be done after parsing all trak because there's no order requirement

libavformat/rtmpproto.c

@@ -761,7 +761,6 @@ static int get_packet(URLContext *s, int for_header)
         }
         ff_rtmp_packet_destroy(&rpkt);
     }
-    return 0;
 }
 
 static int rtmp_close(URLContext *h)

libavformat/swfdec.c

@@ -204,7 +204,6 @@ static int swf_read_packet(AVFormatContext *s, AVPacket *pkt)
     skip:
         avio_skip(pb, len);
     }
-    return 0;
 }
 
 AVInputFormat ff_swf_demuxer = {

libavutil/opt.c

@@ -196,7 +196,6 @@ int av_set_string3(void *obj, const char *name, const char *val, int alloc, cons
                 return 0;
             notfirst=1;
         }
-        return AVERROR(EINVAL);
     }
 
     if (alloc) {