Fix variables shadowing in wallet.cpp
Fix variables shadowing in wallet.cpp

wallet/wallet.cpp:1153:55: warning: declaration shadows a local variable [-Wshadow]
                    map<uint256, int>::const_iterator mi = pwallet->mapRequestCount.find(hashBlock);
                                                      ^
wallet/wallet.cpp:1145:47: note: previous declaration is here
            map<uint256, int>::const_iterator mi = pwallet->mapRequestCount.find(GetHash());
                                              ^
wallet/wallet.cpp:2929:30: warning: declaration shadows a local variable [-Wshadow]
    for (set<CTxDestination> grouping : groupings)
                             ^
wallet/wallet.cpp:2870:25: note: previous declaration is here
    set<CTxDestination> grouping;
                        ^

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