[compiler-rt] Some clean up / refactoring in sanitizer_symbolizer_libcdep.cpp.

Summary:
Nothing critical, just a few potential improvements I've noticed while reading
the code:
- return `false` when symbolizer buffer is too small to read all data
- invert some conditions to reduce indentation
- prefer `nullptr` over `0` for pointers; init some pointers on stack;
- remove minor code duplication

Reviewers: eugenis, vitalybuka

Subscribers: dberris, #sanitizers, llvm-commits, kcc

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D74137