syscall_emul: Corrected a bug on the syscall "read()".

Read() should not write anything when returning 0 (EOF).

This patch does not correct the same bug occuring for :
nbr_read=read(file, buf, nbytes) 
When nbr_read<nbytes, nbytes bytes are copied into the virtual

RAM instead of nbr_read. If buf is smaller than nbytes, a 

page fault occurs, even if buf is in fact bigger than nbr_read.

Works with CERE codelets (https://github.com/benchmark-subsetting/cere),

correcting the issue when replacing dumps in memory.