AST-2018-004: Restrict the number of Accept headers in a SUBSCRIBE.

When receiving a SUBSCRIBE request the Accept headers from it are
stored locally. This operation has a fixed limit of 32 Accept headers
but this limit was not enforced. As a result it was possible for
memory outside of the allocated space to get written to resulting
in a crash.

This change enforces the limit so only 32 Accept headers are
processed.

ASTERISK-27640
Reported By: Sandro Gauci

Change-Id: I99a814b10b554b13a6021ccf41111e5bc95e7301