[compiler-rt] Check for empty buffer in Addr2LineProcess::ReadFromSymbolizer

This fixes a bug in the ReadFromSymbolizer method of the
Addr2LineProcess class; if the input is too large, the returned buffer
will be null and will consequently fail the CHECK. The proposed fix is
to simply check if the buffer consists of only a null-terminator and
return if so (in effect skipping that frame). I tested by running one of
the unit tests both before and after my change.

Submitted on behalf of david-y-lam.

Reviewers: eugenis, alekseyshl, kcc

Reviewed By: alekseyshl

Differential Revision: https://reviews.llvm.org/D36207

llvm-svn: 310089