Fix yet another segfault within the queries loop

tools/dcdbquery/query.cpp

@@ -428,7 +428,9 @@ void DCDBQuery::execute() {
 			continue;
 		    }
 		}
-		it++;
+		if (it != range.second) {
+		    it++;
+		}
 	    }
 
 	    // Query raw values next
@@ -446,7 +448,9 @@ void DCDBQuery::execute() {
 
 	    prevSensorName = q->first.name;
 	}
-	q++;
+	if (q != queries.end()) {
+	    q++;
+	}
     }
 }