Fix style in CMSG_* "macros" for fuchsia

This change addresses two style errors found in PR review:
* indent and linebreak in CMSG_NXTHDR
* prefer `0 as *mut cmsghdr` over `core::ptr::nul_mut()`

Author: amanda-tait

src/fuchsia/mod.rs

@@ -3255,13 +3255,14 @@ f! {
         cmsg.offset(1) as *mut c_uchar
     }
 
-    pub fn CMSG_NXTHDR(mhdr: *const msghdr, cmsg: *const cmsghdr) ->
-        *mut cmsghdr {
+    pub fn CMSG_NXTHDR(mhdr: *const msghdr, cmsg: *const cmsghdr)
+        -> *mut cmsghdr
+    {
         if ((*cmsg).cmsg_len as ::size_t) < ::mem::size_of::<cmsghdr>() {
-            core::ptr::null_mut()
+            0 as *mut cmsghdr
         } else if __CMSG_NEXT(cmsg).add(::mem::size_of::<cmsghdr>())
             >= __MHDR_END(mhdr) {
-            core::ptr::null_mut()
+            0 as *mut cmsghdr
         } else {
             __CMSG_NEXT(cmsg).cast()
         }
@@ -3271,7 +3272,7 @@ f! {
         if (*mhdr).msg_controllen as ::size_t >= ::mem::size_of::<cmsghdr>() {
             (*mhdr).msg_control.cast()
         } else {
-            core::ptr::null_mut()
+            0 as *mut cmsghdr
         }
     }