ffm_write_write_index: check lseek() return code

Fixes CID732185
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffserver.c

@@ -346,7 +346,8 @@ static int ffm_write_write_index(int fd, int64_t pos)
 
     for(i=0;i<8;i++)
         buf[i] = (pos >> (56 - i * 8)) & 0xff;
-    lseek(fd, 8, SEEK_SET);
+    if (lseek(fd, 8, SEEK_SET) < 0)
+        return AVERROR(EIO);
     if (write(fd, buf, 8) != 8)
         return AVERROR(EIO);
     return 8;