[PATCH] [opcodes/arm] Fix potential null pointer dereferences

[Nick Clifton]

Hi Luis,

>    PR 29598: dereferenced null pointer in arm-dis.c
> 
>    As pointed out in the bug ticket, we have a couple potential null pointer
>    dereferencing situations for mapping_symbol_for_insn. Harden those.

Approved - please apply.

Cheers
   Nick