sysdep.c: cprintf() truncation fix (Phil Slack)

sysdep.c: cprintf() truncation fix (Phil Slack)

Original code (sysdep.c) assumed a return code of -1 from vsnprintf()
was a truncation.  Actually, it is an output error and the code still
tried to output it.  A return of the buffer size or larger means it was
truncated.  Changed the processing to handle return values properly.