Ensure that path[0] == '\0' upon entry to delete_content()

As a quick fix, staticise path to ensure that it is zero-filled
upon initial entry to the function.

In rare cases there's a possible buffer overflow and/or path
mess up on the first call to strlcat() with path[0] != '\0'.

The proper fix is to utilize util/recurse.c but the traversal
order cannot be specified at the moment.

Expect a better fix in the future.

switch_root.c

@@ -17,7 +17,7 @@
 static void
 delete_content(const char *dir, dev_t curdevice)
 {
-	char path[PATH_MAX];
+	static char path[PATH_MAX];
 	DIR *d;
 	struct stat st;
 	struct dirent *dent;