[base] Clean up signedness issues in `ftdbgmem.c'. Also fix other minor issues. * src/base/ftdbgmem.c (FT_MemTableRec): Replace all FT_ULong types with FT_Long for consistency. (ft_mem_primes): Change type to `FT_Int'. (ft_mem_closest_prime, ft_mem_table_set): Updated. (ft_mem_debug_panic, ft_mem_debug_alloc, ft_mem_debug_free, ft_mem_debug_realloc): Use `static' keyword and fix signedness warnings where necessary. (ft_mem_table_resize, ft_mem_table_new, ft_mem_table_destroy, ft_mem_table_get_nodep, ft_mem_debug_init, FT_DumpMemory): Fix types and add or remove casts to avoid signedness warnings.