[lldb/crashlog] Fix crash when loading non-symbolicated report

This patch should address the crashes when parsing a the crash report
frame dictionary.

If the crash report is not symbolicated, the `symbolLocation` key will
be missing. In that case, we should just use the `imageOffset`.

rdar://109836386

Differential Revision: https://reviews.llvm.org/D151844



Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>