Reject non-mimepart resources in mailparse_msg_* instead of crashing

[iliaal]

What

Five functions crash the PHP process when handed a resource that is not a mimepart, for example mailparse_msg_parse(tmpfile(), "x").

mailparse_fetch_mimepart_resource expands to zend_fetch_resource, which returns NULL (and raises a TypeError on PHP 8) when the resource type does not match. The functions used the result without checking, so the following call dereferenced NULL: mailparse_msg_parse, mailparse_msg_get_structure, mailparse_msg_get_part_data, mailparse_msg_get_part, and the mailparse_msg_extract_part* family.

The object-method path already NULL-checks the same fetch; only these procedural entry points did not.

Fix

Add if (part == NULL) { RETURN_FALSE; } after each fetch, mirroring the existing guard in mimemsg_get_object. The pending TypeError (PHP 8) or warning (PHP 7) then surfaces cleanly instead of a segfault.

[iliaal]

Superseded by #70, reopened from the fork (iliaal/pecl-mail-mailparse). Branch should not have been pushed to php/ directly.