dummynet: free(NULL, M_DUMMYNET); is safe

There's no need to check pointers for NULL before free()ing them.

No functional change.

MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30382

(cherry picked from commit 1b2dbe37fa32d7255faf7d1feec7bb31414a8102)