don't use delete with malloc on store

[hmaarrfk]

/home/mark/git/OpenJPH/src/core/common/ojph_mem.h: In member function 'void ojph::mem_fixed_allocator::alloc()':
/home/mark/git/OpenJPH/src/core/common/ojph_mem.h:85:20: warning: deleting 'void*' is undefined [-Wdelete-incomplete]
   85 |           delete[] store;
      |                    ^~~~~

[aous72]

Hi Mark,

Good catch; thank you for putting this PR.

Kind regards,
Aous.