[DebugInfo] Don't crash when given invalid DWARFv5 line table prologue.

This patch replaces an assertion with an explicit check for the validity
of the FORM parameters. The assertion was triggered when the DWARFv5
line table contained a zero address size.

This fixes OSS-Fuzz Issue 4644
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=4644

Differential revision: https://reviews.llvm.org/D41615

llvm-svn: 321863