target-i386: cpu: Fix potential buffer overrun in get_register_name_32()

Spotted by Coverity,
x86_reg_info_32[] is CPU_NB_REGS32 elements long, so accessing
x86_reg_info_32[CPU_NB_REGS32] will be one element off array.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: liguang <lig.fnst@cn.fujitsu.com>
Reviewed by: Jesse Larrew <jlarrew@linux.vnet.ibm.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
