To reduce the performance penalty of commit 779ca42a
Ian has recommended to release the socket_mutex during the call of select.
See https://github.com/eclipse/paho.mqtt.c/issues/385#issuecomment-372299490

As a preparation for this, the locking of the socket_mutex is moved inside
the Socket_getReadySocket() function.
Signed-off-by: Juergen Kosel <juergen.kosel@softing.com>