fixed: uninitalized error of valgrind

fixed the valgrind warning:

Conditional jump or move depends on uninitialised value(s)
   at 0x11271E: check_fs (ff.c:3301)
   by 0x1127C7: find_volume (ff.c:3325)
   by 0x1129B0: mount_volume (ff.c:3427)
   by 0x112FAD: f_mount (ff.c:3680)
   by 0x10D7DC: main (main.c:139)

test-in-docker/src/main.c

@@ -129,7 +129,7 @@ int main(int argc, char **argv) {
 		return 1;
 	}
 
-	FATFS fs;
+	FATFS fs = {};
 	unsigned char *buf[4096];
 
 	(void) argc; /* suppress unused warning */