[binutils-gdb] Fix potential illegal memory access in Xtensa port.

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT d84ed528d4817b0ff854006b65a9f6ec75f0407a ***

Author: Max Filippov <jcmvbkbc@gmail.com>
Branch: master
Commit: d84ed528d4817b0ff854006b65a9f6ec75f0407a

Fix potential illegal memory access in Xtensa port.

	* xtensa-isa.c (xtensa_isa_init): Don't update lookup table
	entries for sysregs with negative indices.