fix string concat errors from JDK dir names

I have a number of funkily-named JDKs that aren't meant to be
picked up by fink, which cause output like this:

Use of uninitialized value $java_inc_dir in substitution (s///) at /sw/lib/perl5/Fink/VirtPackage.pm line 410.
Use of uninitialized value $dir in concatenation (.) or string at /sw/lib/perl5/Fink/VirtPackage.pm line 420.
Use of uninitialized value $dir in concatenation (.) or string at /sw/lib/perl5/Fink/VirtPackage.pm line 421.
Use of uninitialized value $dir in concatenation (.) or string at /sw/lib/perl5/Fink/VirtPackage.pm line 427.
Use of uninitialized value $dir in pattern match (m//) at /sw/lib/perl5/Fink/VirtPackage.pm line 433.

This patch tells fink to print a warning (if debug enabled)
and skip those JDKs.