-Werror=maybe-uninitialized on file system/page_storage.h with gcc 9

[claudiocabral]

compiling with gcc-arm-none-eabi-9-2019-q4-major fails with the following error:

./stmlib/system/page_storage.h: In member function 'bool plaits::Settings::Init()':
./stmlib/system/page_storage.h:185:24: error: '*((void*)& h +4)' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  185 |       FLASH_ProgramWord(address, *words);
      |       ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~
./stmlib/system/page_storage.h:171:17: note: '*((void*)& h +4)' was declared here
  171 |     ChunkHeader h;
      |                 ^

The compiler gets confused because ChunkHeader doesn't default intiialize its varibles and fails to see the assignments next to it.
The fix is to use the initializer_list constructor instead of manually assigning the variables