[analyzer] Ignore calculated indices of <= 0 in VLASizeChecker

Summary:
See https://bugs.llvm.org/show_bug.cgi?id=46128. The checker does not
yet comprehend constraints involving multiple symbols, so it's possible
to calculate a VLA size that's negative or 0. A LIT is added to catch
regressions, and this change simply bails if a VLA size of 0 or less is
calculated.

Reviewers: balazske, NoQ, martong, baloghadamsoftware, Szelethus, gamesh411

Reviewed By: balazske, NoQ, Szelethus

Subscribers: xazax.hun, szepet, rnkovacs, a.sidorin, mikhail.ramalho, donat.nagy, Charusso, ASDenysPetrov, cfe-commits, dkrupp

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80903