Fix, workaround for maybe uninitialized variables

Warnings addressed:

mastercore_rpc.cpp:1515:10: warning: ‘valid’ may be used uninitialized
in this function [-Wmaybe-uninitialized]
mastercore_rpc.cpp:1845:9: warning: ‘MPTxTypeInt’ may be used
uninitialized in this function [-Wmaybe-uninitialized]
Merge pull request #179 from dexX7/mcore-warning-gettradessince

Fix, workaround for maybe uninitialized variables