[lldb] Fix a potential bug that may cause assert failure in CommandObject::CheckRequirements

Summary: `CommandObject::CheckRequirements` requires cleaning up `m_exe_ctx`
between commands. Function `HandleOptionCompletion` returns without cleaning up
`m_exe_ctx` could cause assert failure in later `CheckRequirements`.

Reviewers: teemperor, JDevlieghere

Reviewed By: teemperor

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D80447