Revert "avprobe: Zero the allocated avio buffer memory"

This reverts commit 0e0538ae.

The valgrind warning was a false positive due to OSX implementation of
printf (invoking a strnlen), while this code is actually fine, since the
format specifier %.*s guarantes that no more than buf_size bytes from
buf will be read.
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>

avprobe.c

@@ -1039,7 +1039,7 @@ static int probe_buf_write(void *opaque, uint8_t *buf, int buf_size)
 int main(int argc, char **argv)
 {
     int ret;
-    uint8_t *buffer = av_mallocz(AVP_BUFFSIZE);
+    uint8_t *buffer = av_malloc(AVP_BUFFSIZE);
 
     if (!buffer)
         exit(1);