Fix insufficient memory allocation in ESP32 select_thread_loop

Building with ESP-IDF v5.4 emmits the folowing warning:
```
/__w/AtomVM/AtomVM/src/platforms/esp32/components/avm_sys/sys.c: In function 'select_thread_loop':
/__w/AtomVM/AtomVM/src/platforms/esp32/components/avm_sys/sys.c:680:26: warning: allocation of
 insufficient size '0' for type 'struct pollfd' with size '8' [-Walloc-size]
  680 |     struct pollfd *fds = malloc(0);
      |                          ^~~~~~
```

These changes fix the problem by allocating enough memory to hold the struct.

Signed-off-by: Winford <winford@object.stream>

CHANGELOG.md

@@ -27,6 +27,7 @@ certain VM instructions are used.
 - Fixed an issue where a timeout would occur immediately in a race condition
 - Fixed SPI close command
 - Added missing lock on socket structure
+- Fixed insufficient memory allocation in ESP32 select_thread_loop
 
 ## [0.6.5] - 2024-10-15
 

src/platforms/esp32/components/avm_sys/sys.c

@@ -677,7 +677,7 @@ static void *select_thread_loop(void *arg)
 {
     GlobalContext *glb = arg;
     struct ESP32PlatformData *platform = glb->platform_data;
-    struct pollfd *fds = malloc(0);
+    struct pollfd *fds = malloc(sizeof(struct pollfd));
     while (!platform->select_thread_exit) {
         int select_events_poll_count = platform->select_events_poll_count;
         int poll_count = 1;