[GAS] fix crash on erroneous directive

[Nick Clifton]

Hi Nathan,

>> You could define a symbol and then use to the right nop depending on the
>> symbol's value.
>
> like this?

Yes - although it turns out that there are other targets that have nops 
that take arguments, do not have a nop instruction at all, or do not 
generate dwarf2 debug information, so the test as you had it needed a 
little more tweaking.  I have taken care of this however, and checked 
the result in.

Cheers
   Nick