py-common: clean-up: unreachable error handling code

py-common: clean-up: unreachable error handling code

Both `bt_py_common_format_exception()` and `bt_py_common_format_tb()`
use the same error handling pattern of defining an `error` label and
free-ing `msg_buf` if it has been created before jumping there.

In both cases, the creation of `msg_buf` is the last operation that
can fail, which makes the check, and clean-up, unrechable.

From Coverity's report:
1408326 Logically dead code

The indicated dead code may have performed some action; that action
will never occur.

In bt_py_common_format_tb: Code can never be reached because of a
logical contradiction (CWE-561)

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I81b5a2db2cefe1fe08e25819bfa2d8e0fd78ee82
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3154
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>
Tested-by: jenkins <jenkins@lttng.org>