Acquire the global lock before initializing malloc (#410)

In a multi-threaded execution we need to make sure that only exactly one
thread initializes malloc. The function try_init_allocator() can't
easily be made thread-safe, so just move the call to
try_init_allocator() inside the block that holds the lock.

dlmalloc/src/malloc.c

@@ -4593,14 +4593,14 @@ void* dlmalloc(size_t bytes) {
  ensure_initialization(); /* initialize in sys_alloc if not using locks */
 #endif
 
+ if (!PREACTION(gm)) {
 #if __wasilibc_unmodified_upstream // Try to initialize the allocator.
 #else
- if (!is_initialized(gm)) {
- try_init_allocator();
- }
+  if (!is_initialized(gm)) {
+  try_init_allocator();
+  }
 #endif
 
- if (!PREACTION(gm)) {
  void* mem;
  size_t nb;
  if (bytes <= MAX_SMALL_REQUEST) {