Bug 1439236: exit early if m_copym gets called with null ptr r=dminor

Cheery-picked upstream commit:
d89882d049

MozReview-Commit-ID: 36bYbfIaqEz

--HG--
extra : rebase_source : 334b13ec00e9d82346a2c6ebfdadaf87b6161406

netwerk/sctp/src/user_mbuf.c

@@ -987,6 +987,13 @@ m_copym(struct mbuf *m, int off0, int len, int wait)
 
 	KASSERT(off >= 0, ("m_copym, negative off %d", off));
 	KASSERT(len >= 0, ("m_copym, negative len %d", len));
+	KASSERT(m != NULL, ("m_copym, m is NULL"));
+
+#if !defined(INVARIANTS)
+	if (m == NULL) {
+		return (NULL);
+	}
+#endif
 
 	if (off == 0 && m->m_flags & M_PKTHDR)
 		copyhdr = 1;