Fix crash when disassembling invalid range on powerpc vle

Fix crash when disassembling invalid range on powerpc vle

I got a report of a gdb crash for vle and further investigation showed an
attempt to disassemble an invalid memory range.  I tracked the crash down
to the code in get_powerpc_dialect, where we fail to make sure we have a
valid section pointer before dereferencing it.

There is no such problem for rs6000-based disassembling.

opcodes/ChangeLog:

2016-12-08  Luis Machado  <lgustavo@codesourcery.com>

	* ppc-dis.c (get_powerpc_dialect): Check NULL info->section.