ext_mbstring: Fix invalid free() in php_mb_parse_encoding

Summary:
A chunk of memory allocated by 'req::calloc_noptrs' was being freed by 'free'. The former internally calls 'calloc' and returns a pointer at an index sizeof(MallocNode) inside the allocated buffer. This led to freeing invalid memory.

CVE-2019-11930

Reviewed By: jjergus

Differential Revision: D18179908

fbshipit-source-id: 0e3fe77628e0b9dee8361e712b8abac59ae5ed22