Fix uninitialized timed caches

This could prevent resizeable plugins from being resizeable in Ardour if
the initialized timestamp just happened to be greater than the current
time, since the returned result would then likely be some invalid value.

src/common/utils.h

@@ -278,5 +278,5 @@ class TimedValueCache {
 
    private:
     T value;
-    time_t valid_until;
+    time_t valid_until = 0;
 };