libcpu: riscv_disasm use 50 char mnebuf

libcpu: riscv_disasm use 50 char mnebuf

Some "illegal" instructions can be up to 24 chars (192 bits), We'll
print this as 0x<48 hex chars>. So make sure the mnebuf is 50 chars
(no terminating zero is needed).

This shows up with _FORTIFY_SOURCE which would immediate terminate on
such "illegal" instructions. Without we just use a few extra bytes on
the stack (which aren't used afterwards, without any issue, even
though it is technically UB).

	* libcpu/riscv_disasm.c (riscv_disasm): Extend char mnebuf
        array to 50.

Signed-off-by: Mark Wielaard <mark@klomp.org>