Some illegal instructions are capable of being executed

Created by: zarubaf

Location: cva6/src/decoder.sv (https://github.com/openhwgroup/cva6/blob/master/src/decoder.sv), line 153 to 162.

Details: The if statement in the default case is missing else. Hence the illegal_instr will not be set even for instructions not meeting the if condition.

Thus any illegal instruction of the format 64’hxxx00073 (‘x’ is dont-care) will not be detected as illegal by Ariane.