disas: cris: Fix 0 buffer length case

Cris has the complication of variable length instructions and has
a check in place to clamp memory reads in case the disas request
doesn't have enough bytes for the instruction being disas'd. This
breaks down in the case where disassembling for the monitor where
the buffer length is defaulted to 0.

The buffer length should never be zero for a regular target_disas,
so we can safely assume the 0 case is for the monitor in which case
consider the buffer length to be the max for cris instructions.

Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>