BUG/MINOR: proxy: avoid NULL-deref in post_section_px_cleanup()

BUG/MINOR: proxy: avoid NULL-deref in post_section_px_cleanup()

post_section_px_cleanup(), which was implemented in abcc73830
("MEDIUM: proxy: register a post-section cleanup function"), is called
for the current section no matter if the parsing was aborted due to
a fatal error. In this case, the curproxy pointer may point to NULL,
yet post_section_px_cleanup() assumes curproxy pointer is always valid,
which could lead to NULL-deref.

For instance, the config below will cause SEGFAULT:

  listen toto titi

To fix the issue, let's simply consider that the curproxy pointer may
be NULL in post_section_px_cleanup(), in which case we skip the cleanup
for the curproxy since there is nothing we can do.

No backport needed