Fix memory leak in edithost

The problem is that when the parameter 'pat' is null, the function locally
allocates a NULL string but never frees it.

Instead of tracking the local alloc, it is noted that the while(*pat) never
enters when there is a local alloc.
So instead of doing the local alloc, check that 'pat' is null before the
while(*pat) loop.

Found using clang's static analyzer - scan-build

Submitted by: Thomas Rix <trix@juniper.net>
Reviewed by: markm
Approved by: sjg (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D9689