[lldb/crashlog] Fix exception signal parsing

In some cases, it can happen that crashlogs don't have any signal in
the exception, which causes the parser to crash.

This fixes the parsing by checking if the `signal` field is in the
`exception` dictionary before trying to access it.

rdar://84552251

Differential Revision: https://reviews.llvm.org/D119504



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>