safer, more robust file system access in flash I/O
safer, more robust file system access in flash I/O

Cleaned up type conversion in load_flash() and save_flash() between signed and unsigned long integers (fixes a couple warnings found by macro FLASH_SIZE being an unsigned literal constant, reported by the Pacific C compiler for MS-DOS).  Added type range checks using <limits.h> to analyze the validity of bytes_read and bytes_sent as a file size of (long) type before explicitly casting to a signed long type that will comply with the C standard for file sizes as signed integers used by ftell() and fseek().

cxd4/zs-flash,cxd4/zs-flash