Add more documentation in PJSIP's parser to prevent stack buffer overflow

[ming]

This ticket will add more documentation in the description of the APIs in pjsip's sip_parser.c, in particular the requirement that the buffer must be NULL terminated and the size parameter must not include the NULL terminator.

Also in the ticket:

• Add assertion in pjlib-util's scanner to ensure that the buffer is NULL terminated.
  If when using tools such as Valgrind or AddressSanitizer, you get a buffer read overflow warning in this assertion line, it means that you incorrectly passed the buffer length (it should be buffer length - 1).
• Add robustness check in sip_multipart to prevent buffer overflow