0-initialize thread-specific data upon thread creation. (#508)

If thread-specific data is not set to 0 upon thread creation,
`__pthread_tsd_run_dtors` will end up running destructors passing
uninitialized memory as memory addresses, which can lead to memory
corruption.

This issue can be triggered when malloc() returns a memory address that
was freed before, as in that case memory is not zeroed out.

libc-top-half/musl/src/thread/pthread_create.c

@@ -458,6 +458,9 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
  if (!map) goto fail;
 #endif
  tsd = map + size - __pthread_tsd_size;
+#ifndef __wasilibc_unmodified_upstream
+ memset(tsd, 0, __pthread_tsd_size);
+#endif
  if (!stack) {
 #ifdef __wasilibc_unmodified_upstream
  stack = tsd - libc.tls_size;