fftools/ffmpeg: Check read() for failure

Fixes: CID1591932 Ignoring number of bytes read

Sponsored-by: Sovereign Tech Fund
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 34fd247c3b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

fftools/ffmpeg.c

@@ -306,8 +306,9 @@ static int read_key(void)
         }
         //Read it
         if(nchars != 0) {
-            read(0, &ch, 1);
-            return ch;
+            if (read(0, &ch, 1) == 1)
+                return ch;
+            return 0;
         }else{
             return -1;
         }