Fix Coverity issue 1212346

Coverity was (I believe) correctly complaining about a possible null
dereference during generic resolution.  I looked through the filterCandidates
call heirarchy and it struck me as possible (though very unlikely) for a
control flow to reach that point when best was non-null but best->fn was.  To
fix, I added a check to best->fn to ensure it was non-null before the access
was performed.
Add a comment to the change.

Tom pointed out that this extra check probably means we should restructure
resolveNormalCall.  This isn't a high priority for me, but it seems good to
note, so I added a note.
Merge branch 'master' into coverity1212346
Merge pull request #572 from lydia-duncan/coverity1212346

Fix Coverity issue 1212346 [reviewed by hilde]

Coverity was (I believe) correctly complaining about a possible null
dereference during generic resolution. I looked through the filterCandidates
call heirarchy and it struck me as possible (though very unlikely) for
control flow to reach that point when best was non-null but best->fn was. To
fix, I added a check to best->fn to ensure it was non-null before the access
was performed.