cmsg.3: Note that CMSG_FIRSTHDR can return NULL

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/cmsg.3

@@ -78,6 +78,9 @@ returns a pointer to the first
 in the ancillary
 data buffer associated with the passed
 .IR msghdr .
+It returns NULL if there isn't enough space for a
+.I cmsghdr
+in the buffer.
 .IP *
 .BR CMSG_NXTHDR ()
 returns the next valid