Tidy implementation and use of QDateTimeParser::setDigit()

The implementation did a validity check on data it then gave to
constructors that repeat the validity checks; so do the construction
first and use the cheaper instance isValid() methods instead.

Fixed local callers (but not QDateTimeEditPrivate::stepBy, the only
other caller) to actually attend to the return value; if the attempt
to set a field fails, it probably means the min is too low, or max too
high; and comparing the modified date-time against global min and max
makes no sense if it hasn't been revised anyway.

Change-Id: If4505c43c92e247445dcd10ac436b775c3ead4da
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qdatetimeparser.cpp

@@ -179,11 +179,14 @@ bool QDateTimeParser::setDigit(QDateTime &v, int index, int newVal) const
             day = max;
         }
     }
-    if (QDate::isValid(year, month, day) && QTime::isValid(hour, minute, second, msec)) {
-        v = QDateTime(QDate(year, month, day), QTime(hour, minute, second, msec), spec);
-        return true;
-    }
-    return false;
+
+    const QDate newDate(year, month, day);
+    const QTime newTime(hour, minute, second, msec);
+    if (!newDate.isValid() || !newTime.isValid())
+        return false;
+
+    v = QDateTime(newDate, newTime, spec);
+    return true;
 }
 
 
@@ -1765,16 +1768,13 @@ bool QDateTimeParser::skipToNextSection(int index, const QDateTime &current, con
 
     QDateTime tmp = current;
     int min = absoluteMin(index);
-    setDigit(tmp, index, min);
-    if (tmp < minimum) {
+    if (!setDigit(tmp, index, min) || tmp < minimum)
         min = getDigit(minimum, index);
-    }
 
     int max = absoluteMax(index, current);
-    setDigit(tmp, index, max);
-    if (tmp > maximum) {
+    if (!setDigit(tmp, index, max) || tmp > maximum)
         max = getDigit(maximum, index);
-    }
+
     int pos = cursorPosition() - node.pos;
     if (pos < 0 || pos >= text.size())
         pos = -1;