fix MSAN

Author: corona10

Objects/mimalloc/prim/unix/prim.c

@@ -124,7 +124,7 @@ static bool unix_detect_overcommit(void) {
   #if defined(__linux__)
     int fd = mi_prim_open("/proc/sys/vm/overcommit_memory", O_RDONLY);
     if (fd >= 0) {
-      char buf[32];
+      char buf[32] = {0};
       ssize_t nread = mi_prim_read(fd, &buf, sizeof(buf));
       mi_prim_close(fd);
       // <https://www.kernel.org/doc/Documentation/vm/overcommit-accounting>

Objects/mimalloc/random.c

@@ -178,7 +178,7 @@ uintptr_t _mi_os_random_weak(uintptr_t extra_seed) {
 }
 
 static void mi_random_init_ex(mi_random_ctx_t* ctx, bool use_weak) {
-  uint8_t key[32];
+  uint8_t key[32] = {0};
   if (use_weak || !_mi_prim_random_buf(key, sizeof(key))) {
     // if we fail to get random data from the OS, we fall back to a
     // weak random source based on the current time