Say which height we are at when we get an assertion from GetBlockHash

src/chain.h

@@ -512,7 +512,10 @@ public:
 
     uint256 GetBlockHash() const
     {
-        assert(phashBlock);
+        if(phashBlock == NULL) {
+            fprintf(stderr, "GetBlockHash: phashBlock NULL at height %d\n", this->GetHeight());
+            assert(phashBlock);
+        }
         return *phashBlock;
     }