[CSSPGO][llvm-profgen] Truncate stack samples with invalid return address.

[CSSPGO][llvm-profgen] Truncate stack samples with invalid return address.

Invalid frame addresses exist in call stack samples due to bad unwinding. This could happen to frame-pointer-based unwinding and the callee functions that do not have the frame pointer chain set up. It isn't common when the program is built with the frame pointer omission disabled, but can still happen with third-party static libs built with frame pointer omitted.

Reviewed By: wenlei

Differential Revision: https://reviews.llvm.org/D109638