[lldb] Fix double free in python bindings error handling.

If we have a `%typemap(freearg)` that frees the argument, we shouldn't
free it manually on an error path before calling `SWIG_fail`.
`SWIG_fail` will already free the memory in this case, and doing it
manually results in a double free.

Differential Revision: https://reviews.llvm.org/D147007