[GWP-ASan] Crash handler API returns sizeof(collected trace)

Summary:
Fix up a slight bug with the crash handler API, where we say that we
return the size of the collected trace (instead of the size of the trace
that's returned) when the return buffer is too small, and the result is
truncated.

Also, as a result, patch up a small uninitialized memory bug.

Reviewers: morehouse, eugenis

Reviewed By: eugenis

Subscribers: #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D84652