Perl: handling of allocation errors.

Perl: handling of allocation errors.

Previously, allocation errors in nginx.xs were more or less ignored,
potentially resulting in incorrect code execution in specific low-memory
conditions.  This is changed to use ctx->error bit and croak(), similarly
to how output errors are now handled.

Note that this is mostly a cosmetic change, as Perl itself exits on memory
allocation errors, and hence nginx with Perl is hardly usable in low-memory
conditions.