devel/avarice: unbreak build under 11.2-RELEASE (and clang 6)

[leres]

Summary

Proposed commit message:

After upgrading to 11.2-RELEASE devel/avarice fails to
compile with:

jtagrw.cc:134:13: error: cannot initialize return object of type 'uchar *' (aka 'unsigned char *') with an rvalue of type 'bool'

            return false;
                   ^~~~~

examination of the code shows another case with a similar
check that returns NULL.

I also filed an upstream bug report:

https://sourceforge.net/p/avarice/bugs/24/

PR: 229376
Reviewed by: ? (mentor)
Approved by: joerg (maintainer), ? (mentor)
Differential Revision: ?

[ler]

LGTM