core/shared/platform: Zero memory returned by os_mmap in some platforms (#3551)

The os_mmap should zero the mapped memory like what posix mmap does.

core/shared/platform/rt-thread/rtt_platform.c

@@ -200,7 +200,12 @@ os_cond_wait(korp_cond *cond, korp_mutex *mutex)
 void *
 os_mmap(void *hint, size_t size, int prot, int flags, os_file_handle file)
 {
-    return rt_malloc(size);
+    void *addr;
+
+    if ((addr = rt_malloc(size)))
+        memset(addr, 0, size);
+
+    return addr;
 }
 
 void
@@ -221,4 +226,4 @@ os_dcache_flush(void)
 
 void
 os_icache_flush(void *start, size_t len)
-{}
+{}