win: fix compiler warnings

- udp.c: Fix signedness warning in ternary operator by adding explicit cast to ensure consistent types
- util.c: Fix signed/unsigned comparison warning in loop condition by casting sizeof result to unsigned int
- win.h: Add missing braces in UV_ONCE_INIT to fix -Wmissing-braces warning

include/uv/win.h

@@ -279,7 +279,7 @@ typedef struct {
   DWORD tls_index;
 } uv_key_t;
 
-#define UV_ONCE_INIT { 0, NULL }
+#define UV_ONCE_INIT { 0, { NULL } }
 
 typedef struct uv_once_s {
   unsigned char unused;

src/win/udp.c

@@ -1158,7 +1158,7 @@ int uv__udp_try_send2(uv_udp_t* handle,
   for (i = 0; i < (int) count; i++) {
     r = uv_udp_try_send(handle, bufs[i], nbufs[i], addrs[i]);
     if (r < 0)
-      return i > 0 ? i : r;  /* Error if first packet, else send count. */
+      return i > 0 ? i : (int) r;  /* Error if first packet, else send count. */
   }
 
   return i;

src/win/util.c

@@ -521,7 +521,7 @@ unsigned int uv_available_parallelism(void) {
    */
   count = 0;
   if (GetProcessAffinityMask(GetCurrentProcess(), &procmask, &sysmask))
-    for (i = 0; i < 8 * sizeof(procmask); i++)
+    for (i = 0; i < (unsigned int) (8 * sizeof(procmask)); i++)
       count += 1 & (procmask >> i);
 
   if (count > 0)