Fix memory leak when a plain parameter repeats an RFC2231 name

[iliaal]

What

A plain MIME parameter whose name matches an active RFC2231 continuation leaks the parameter-name string. The leak size is the attacker-controlled name length and it repeats per occurrence, so a crafted header stream is a slow memory-exhaustion vector.

Example: Content-Type: text/plain; URL*0="a"; URL="b". The URL="b" parameter shares the base name of the URL*0 continuation; its emalloc'd name is neither stored nor freed.

Fix

In php_mimeheader_alloc_from_tok, when the active RFC2231 name matches the new plain parameter, free the duplicate name. The guard is name != name_buf so it only frees a separate allocation, never the live name_buf that the first encoded parameter aliases (which would be a use-after-free on the next iteration).

[iliaal]

Superseded by #71, reopened from the fork (iliaal/pecl-mail-mailparse). Branch should not have been pushed to php/ directly.