[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