ffmpeg was computing the frame length of avis via doubles, but was

not properly ceiling them, sometimes resulting in truncation errors.

(merge from render25 branch, commit with revision 28901 by Joe)

source/blender/imbuf/intern/anim.c

@@ -589,9 +589,9 @@ static int startffmpeg(struct anim * anim) {
 	anim->duration = pFormatCtx->duration * pCodecCtx->frame_rate 
 		/ pCodecCtx->frame_rate_base / AV_TIME_BASE;
 #else
-	anim->duration = pFormatCtx->duration 
+	anim->duration = ceil(pFormatCtx->duration
 		* av_q2d(pFormatCtx->streams[videoStream]->r_frame_rate) 
-		/ AV_TIME_BASE;
+		/ AV_TIME_BASE);
 
 #endif
 	anim->params = 0;