Fix: make CCoinsViewDbCursor::Seek work for missing keys

Thanks to Suhas Daftuar for figuring this out.

src/txdb.cpp

@@ -98,7 +98,11 @@ CCoinsViewCursor *CCoinsViewDB::Cursor() const
        that restriction.  */
     i->pcursor->Seek(DB_COINS);
     // Cache key of first record
-    i->pcursor->GetKey(i->keyTmp);
+    if (i->pcursor->Valid()) {
+        i->pcursor->GetKey(i->keyTmp);
+    } else {
+        i->keyTmp.first = 0; // Make sure Valid() and GetKey() return false
+    }
     return i;
 }