DNP3: fix reachable assertion and buffer over-read/overflow.

A DNP3 packet using a link layer header that specifies a zero length can
trigger an assertion failure if assertions are enabled.  Assertions are
enabled unless Bro is compiled with the NDEBUG preprocessor macro
defined.  The default configuration of Bro will define this macro and so
disables assertions, but using the --enable-debug option in the
configure script will enable assertions.  When assertions are disabled,
or also for certain length values, the DNP3 parser may attempt to pass a
negative value as the third argument to memcpy (number of bytes to copy)
and result in a buffer over-read or overflow.

Reported by Travis Emmert.