Fix audit issue 5.3 Crash by Malicious Operator

[pavelkrolevets]

Description:
When processing a message of type ReshareExchangeMessageType from another operator relayed
by the initiator, the operator dereferences the DKGData.reshare field to access the list of new
operators. These types of messages are only expected during reshare DKG and not during the initial
DKG. As a consequence, a malicious operator could intentionally submit an reshare message. This
would cause other operators to dereference a nil pointer and crash.

Solution:

• add checks that objects DKGData.reshare or DKGData.reshare not nil when processing messages.