qemu-img bench: Fix uninitialised writethrough mode

If no -t option is specified, bool writethrough stayed uninitialised.
Initialise it as false, which makes cache=writeback the default cache
mode.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

qemu-img.c

@@ -3570,7 +3570,7 @@ static int img_bench(int argc, char **argv)
     BlockBackend *blk = NULL;
     BenchData data = {};
     int flags = 0;
-    bool writethrough;
+    bool writethrough = false;
     struct timeval t1, t2;
     int i;