add 2 constructors in CDiskBlockPos to simplify class usage

- add a default-constructor, which simply calls SetNull() and a
  constructor to directly pass nFile and nPos
- change code to use that new constructors

src/main.h

@@ -192,6 +192,15 @@ public:
         READWRITE(VARINT(nPos));
     )
 
+    CDiskBlockPos() {
+        SetNull();
+    }
+
+    CDiskBlockPos(int nFileIn, unsigned int nPosIn) {
+        nFile = nFileIn;
+        nPos = nPosIn;
+    }
+
     friend bool operator==(const CDiskBlockPos &a, const CDiskBlockPos &b) {
         return (a.nFile == b.nFile && a.nPos == b.nPos);
     }
@@ -1493,8 +1502,7 @@ public:
         if (nStatus & BLOCK_HAVE_DATA) {
             ret.nFile = nFile;
             ret.nPos  = nDataPos;
-        } else
-            ret.SetNull();
+        }
         return ret;
     }
 
@@ -1503,8 +1511,7 @@ public:
         if (nStatus & BLOCK_HAVE_UNDO) {
             ret.nFile = nFile;
             ret.nPos  = nUndoPos;
-        } else
-            ret.SetNull();
+        }
         return ret;
     }