Fix ignoring tx data requests when fPauseSend is set on a peer

This resolves a bug introduced in
66aa1d58a1 where, if when responding
to a series of transaction requests in a getdata we hit the send
buffer limit and set fPauseSend, we will skip one transaction per
call to ProcessGetData.

Bug found by Cory Fields (@theuni).

Github-Pull: #12392
Rebased-From: c4af7387634765d254d1432746385cf35917d367
Tree-SHA512: d2f7707eb9f925a655f66e5e77ce406c5266f7b2feccd5bcdabf6d5bc27a3f6578e753fac83d9c8c3fd7cf7de6fee086eee2f95f77af99ea2c4e5ae77c322c58

src/net_processing.cpp

@@ -1225,10 +1225,10 @@ void static ProcessGetData(CNode* pfrom, const Consensus::Params& consensusParam
         }
     } // release cs_main
 
-    if (it != pfrom->vRecvGetData.end()) {
+    if (it != pfrom->vRecvGetData.end() && !pfrom->fPauseSend) {
         const CInv &inv = *it;
-        it++;
         if (inv.type == MSG_BLOCK || inv.type == MSG_FILTERED_BLOCK || inv.type == MSG_CMPCT_BLOCK || inv.type == MSG_WITNESS_BLOCK) {
+            it++;
             ProcessGetBlockData(pfrom, consensusParams, inv, connman, interruptMsgProc);
         }
     }