rr: Fix a buffer mgm bug during strict routing callbacks
While doing strict routing (e.g. the R-URI host is a local domain) and
executing the module callbacks, the rr module would provide some bogus
memory pointer as the result of the rr_api.get_route_param() function.
Although opensips would not crash due to the dangling pointer residing
in pkg memory, some callbacks would misbehave (e.g. uac_auth() would
fail to adjust the outbound ACK CSeq, since it cannot locate the ";aci"
Route header field parameter within the dangling pkg area).