res_pjsip_session: sdp group:BUNDLE attribute being truncated

When setting/appending the media id's to the bundle group attribute a '-1' was
being passed to the 'ast_str_set/append' function for the 'max_len' parameter.
This essentially capped the length of the string to what it was originally
allocated with. In this case 64 bytes.

This patch makes it so a '0' is passed as in for the 'max_len', which means
"no maximum length".

ASTERISK-27955 #close

Change-Id: Iec565df6600401d54a502854a53d19bb4cc34876