check result of modbus_set_slave()

[r00t-]

libmodbus considers some slave addresses invalid.
it will report an error, but mbpoll previously ignored this, leading to a crash with an assertion failure.

[r00t-]

before this change:

[~/mbpoll]# ./mbpoll -m rtu -b 9600 -P none /dev/ttyUSB0 -a 248 -r 1 -c 1 -t 3 -u -v
[...]
Slave configuration...: address = 248, report slave id
Communication.........: /dev/ttyUSB0,       9600-8N1 
                        t/o 1.00 s, poll rate 1000 ms
mbpoll: modbus-rtu.c:94: _modbus_rtu_build_request_basis: Assertion `ctx->slave != -1' failed.
Aborted

after:

[~/mbpoll]# ./mbpoll -a 248 -b 9600 -t 3 -r 1 -c 2 /dev/ttyUSB0 
[...]
Slave configuration...: address = [248]
                        start reference = 1, count = 2
Communication.........: /dev/ttyUSB0,       9600-8E1 
                        t/o 1.00 s, poll rate 1000 ms
Data type.............: 16-bit register, input register table

./mbpoll: setting slave address failed: Invalid argument.

[r00t-]

the same crash can be seen in #75