Avoid references to disassociated POINTERs in examples

Handle the expected disassociated pointers so that NAG with -C=pointer does not rightfully complain. Also, update the respective CI jobs to build with the -C=all flag.

examples/example.f90

@@ -166,7 +166,11 @@ CONTAINS
     CALL deallocateTimedelta(mtime_td)
 
     mtime_td => newTimedelta("PT7536.0003S")
-    CALL timedeltatostring(mtime_td, td_string)
+    IF (ASSOCIATED(mtime_td)) THEN
+      CALL timedeltatostring(mtime_td, td_string)
+    ELSE
+      td_string = "<null>"
+    END IF
     WRITE (0, *) "PT7536.0003S (expect <null>) = ", TRIM(td_string)
     CALL deallocateTimedelta(mtime_td)