Fixes to SYS_READ changes in PR #727 per review. (#729)

* Use local variable for read_result instead of *ret, and fix
calculation of *ret for EOF case.

* Found a problem when reading an odd (%4) number of bytes at the end
of a file.  fread (on stm32) get them (say 3 bytes), then askes for
more.  do_semihosting gets a read return of 0 and tries to write that.
mem_write alters the address to be aligned and overwrites then 3 bytes
from the last read.

This change simply tells mem_write to do nothing if len is 0.

* Fix Issues from Fabien-Chouteau's review of my previous patch in isue #727.

* Revert change to mem_write() so it does not confuse fixes to do_semihosting().

* Add cast to avoid warning.