Potential fix for spv client getdata protocol

The getdata function loads blocks from the disk which includes auxpow
blocks, in getdata if the type is MSG_FILTERED_BLOCK.

However when it created a merkle block in that clause it doesnt assign
auxpow to the block and later in the checkproofofwork it tries to access
the auxpow block and probably crashes since its null... I initially did
a hack in the auxpow serialize routines which simply masked the problem
with auxpow.get() != null.. because syncing with android client *SPV
STORE* it was crashing my daemon on getdata requests... could this be
why?