nksip_router:resend_worklist function might cause calling process to hang for gen_server:call timeout

[pickme467]

The scenario:

1. There is sip dialog opened
2. SIP BYE arrives and sip_bye callback is being called
3. Client notified by sip_bye callback calls nksip_request:reply
4. nksip_router sends work to its nksip_call_srv worker process
5. Meanwhile dialog is closed and nksip_call_srv process stops without
   notifying nksip_router that it started processing the next work
6. nksip_router receives 'DOWN' message from stopped process and
   calls nksip_router:resend_worklist
7. nksip_call_srv:find_call is executed, but it doesn't check if found
   Pid is of alive process, it just checks that it is the valid pid
8. nksip_router sends work as gen_server:cast to not exisnting
   process, so it will never call gen_server:reply to calling process,
   causing calling process to wait till timeout.

The solution:
make nksip_call_srv:find_call function to check if process is alive