Change signed->unsigned at 3 code sites

This resolves signed/unsigned comparison warnings.

src/db.cpp

@@ -648,7 +648,7 @@ bool CTxDB::LoadBlockIndex()
                             }
                     }
                     // check level 4: check whether spent txouts were spent within the main chain
-                    int nOutput = 0;
+                    unsigned int nOutput = 0;
                     if (nCheckLevel>3)
                     {
                         BOOST_FOREACH(const CDiskTxPos &txpos, txindex.vSpent)

src/net.cpp

@@ -498,7 +498,7 @@ void ThreadSocketHandler2(void* parg)
 {
     printf("ThreadSocketHandler started\n");
     list<CNode*> vNodesDisconnected;
-    int nPrevNodeCount = 0;
+    unsigned int nPrevNodeCount = 0;
 
     loop
     {

src/util.h

@@ -579,9 +579,9 @@ template <typename T> class CMedianFilter
 private:
     std::vector<T> vValues;
     std::vector<T> vSorted;
-    int nSize;
+    unsigned int nSize;
 public:
-    CMedianFilter(int size, T initial_value):
+    CMedianFilter(unsigned int size, T initial_value):
         nSize(size)
     {
         vValues.reserve(size);