relaypriority calculation error

CCoinsViewCache::GetPriority has an overflow bug that affects the relaypriority calculation. The way the arithmetic works in that function is

dResult += coins->vout[txin.prevout.n].nValue * (nHeight-coins->nHeight); 

the nValue variable type is int64_t, and the int64_t max value is 9223372036854775807 for example

nValue value  is 40000000000000
(nHeight->coins - nHeight )value is 300000 
40000000000000 * 300000 > 9223372036854775807 

so the dResult will be negative number! and the AllowFree function will return False, and the high priority transaction with low fee will be reject by node with error insufficient priority!

I found this bug when I send a transaction with rpc api sendrawtransaction,i debug the source code find it.

and int64_t convert double also will loss accuracy!

@maiiz Thanks for reporting. Are you working on a fix for that particular overflow?

@jonasschnelli i just make a simple pull request for this issues, this my first commit~~

Just cast to double inside the expression... 400,000 BTC in a free transaction, must be nice! :P (in the bitcoin network there really is no free transactions anymore, and I expect all the priority code will be removed soon, but while its there no reason to not fix it.)