windows: avoid heap allocation

The windows function works on a pointer of known size,
so no allocation is required. This also fixes the leak.

Also use the winapi type (which actually is u32).

src/target/windows.rs

@@ -6,6 +6,7 @@ use std::slice::from_raw_parts;
 
 use libc::{free, malloc, wchar_t, wcslen};
 use winapi::{
+    ctypes::c_ulong,
     shared::{
         ws2def::{AF_UNSPEC, SOCKADDR_IN},
         ws2ipdef::SOCKADDR_IN6,
@@ -264,12 +265,11 @@ fn ipv4_addr_equal(sockaddr1: &*mut SOCKADDR_IN, sockaddr2: &*mut SOCKADDR_IN) -
 /// An implementation of `GetIpAddrTable` to get all available network interfaces would be required
 /// in order to support previous versions of Windows.
 fn make_ipv4_netmask(unicast_address: &*mut IP_ADAPTER_UNICAST_ADDRESS_LH) -> Netmask<Ipv4Addr> {
-    let mask = unsafe { malloc(size_of::<u32>()) as *mut u32 };
+    let mut mask: c_ulong = 0;
     let on_link_prefix_length = unsafe { (*(*unicast_address)).OnLinkPrefixLength };
-
-    unsafe { ConvertLengthToIpv4Mask(on_link_prefix_length as u32, mask) };
-
-    let mask = unsafe { *mask };
+    unsafe {
+        ConvertLengthToIpv4Mask(on_link_prefix_length as u32, &mut mask as *mut c_ulong);
+    }
 
     if cfg!(target_endian = "little") {
         // due to a difference on how bytes are arranged on a