Fix a floating point exception for invalid framerate, fixes issue 2470.

Patch by Daniel Kang, daniel.d.kang at gmail

Originally committed as revision 26188 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/yuv4mpeg.c

@@ -316,7 +316,7 @@ static int yuv4_read_header(AVFormatContext *s, AVFormatParameters *ap)
             pix_fmt = alt_pix_fmt;
     }
 
-    if (raten == 0 && rated == 0) {
+    if (raten <= 0 || rated <= 0) {
         // Frame rate unknown
         raten = 25;
         rated = 1;