Initialize all members in Interval constructor.

Fixes memory conditional jumps depending on uninitialized values
and as a consequence the unstable results from qgis_expressiontest

Closes #6877

src/core/qgsexpression.h

@@ -358,7 +358,7 @@ class CORE_EXPORT QgsExpression
         static const int HOUR = 60 * 60;
         static const int MINUTE = 60;
       public:
-        Interval( double seconds = 0 ) { mSeconds = seconds; }
+        Interval( double seconds = 0 ): mSeconds(seconds), mValid(true) { }
         ~Interval();
         double years() { return mSeconds / YEARS;}
         double months() { return mSeconds / MONTHS; }