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.

libraries/ESP8266WiFi/src/BearSSLHelpers.cpp

@@ -234,6 +234,8 @@ namespace brssl {
     if (po) {
       free(po->name);
       free(po->data);
+      po->name = nullptr;
+      po->data = nullptr;
     }
   }