mkcomposefs: Switch to declare-and-initialize to silence clang-analyzer

clang-analyzer complains about a leak; I think it's wrong, but changing
things to atomically initialize the variables silences it.

Signed-off-by: Colin Walters <walters@verbum.org>

tools/mkcomposefs.c

@@ -89,16 +89,13 @@ static int ensure_dir(const char *path, mode_t mode)
 
 static int mkdir_parents(const char *pathname, int mode)
 {
-	cleanup_free char *fn = NULL;
-	char *p;
-
-	fn = strdup(pathname);
+	cleanup_free char *fn = strdup(pathname);
 	if (fn == NULL) {
 		errno = ENOMEM;
 		return -1;
 	}
 
-	p = fn;
+	char *p = fn;
 	while (*p == '/')
 		p++;