blockdev-util: let's initialize return parameter on success

We document the rule that return values >= 0 of functions are supposed
to indicate success, and that in case of success all return parameters
should be initialized. Let's actually do so.

Just a tiny coding style fix-up.

src/basic/blockdev-util.c

@@ -81,6 +81,7 @@ int get_block_device(const char *path, dev_t *dev) {
         if (F_TYPE_EQUAL(sfs.f_type, BTRFS_SUPER_MAGIC))
                 return btrfs_get_block_device(path, dev);
 
+        *dev = 0;
         return 0;
 }