Fix minor GCC10 static analyzer warnings (#7255)

Add minor NULL and double-free checks to source, identified using GCC10
pre-release static `-fanalyzer` on the coude.  These are harmless to
other versions.

Also add explicit include of stdint to Schedule.h, because libstdc++20
will not automatically include it.  Safe and no-op on earlier versions.

cores/esp8266/Schedule.h

@@ -2,6 +2,7 @@
 #define ESP_SCHEDULE_H
 
 #include <functional>
+#include <stdint.h>
 
 #define SCHEDULED_FN_MAX_COUNT 32