funding: avoid dereferencing a nil localDiscoverySignal
funding: avoid dereferencing a nil localDiscoverySignal

This commit fixes a possible panic within the funding manger’s workflow
for pending channels. We now ensure that the local discovery signal
retrieved from the localDiscoverySignals map is always non-nil.
Otherwise, we risk a server panic in the case that a node retransmits
the FundingLocked message after a channel has been fully processed, or
even just sends a FundingLocked message for a non-existent channel.

AndrewSamokhvalov/lnd,LightningNetwork/lnd,ddustin/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,aakselrod/lnd,ddustin/lnd,LightningNetwork/lnd