Fix malloc non-thread-safe usage in lib-wasi-threads test (#2022)

In the WASI thread test modified in this PR, malloc was used in multiple threads
without a lock. But wasi-libc implementation of malloc is not thread-safe.

core/iwasm/libraries/lib-wasi-threads/test/update_shared_data_and_alloc_heap.c

@@ -11,6 +11,7 @@
 #include <stdio.h>
 #include <assert.h>
 #include <stdbool.h>
+#include <pthread.h>
 
 #include "wasi_thread_start.h"
 
@@ -29,6 +30,7 @@ typedef struct {
     int *pval;
 } shared_t;
 
+pthread_mutex_t mutex;
 int *vals[NUM_THREADS];
 
 void
@@ -39,7 +41,9 @@ __wasi_thread_start_C(int thread_id, int *start_arg)
     for (int i = 0; i < NUM_ITER; i++)
         __atomic_fetch_add(data->count, 1, __ATOMIC_SEQ_CST);
 
+    pthread_mutex_lock(&mutex); /* malloc is not thread-safe in wasi-libc */
     vals[data->iteration] = malloc(sizeof(int));
+    pthread_mutex_unlock(&mutex);
     *vals[data->iteration] = data->iteration;
 
     __atomic_store_n(&data->th_done, 1, __ATOMIC_SEQ_CST);
@@ -53,6 +57,9 @@ main(int argc, char **argv)
     int thread_ids[NUM_THREADS];
     int *count = calloc(1, sizeof(int));
 
+    assert(count != NULL && "Failed to call calloc");
+    assert(pthread_mutex_init(&mutex, NULL) == 0 && "Failed to init mutex");
+
     for (int i = 0; i < NUM_THREADS; i++) {
         assert(start_args_init(&data[i].base)
                && "Stack allocation for thread failed");
@@ -82,5 +89,7 @@ main(int argc, char **argv)
     }
 
     free(count);
+    assert(pthread_mutex_destroy(&mutex) == 0 && "Failed to destroy mutex");
+
     return EXIT_SUCCESS;
 }