i#1686 arm decoder testing: TYPE_NI_b8_b0 bug fix
i#1686 arm decoder testing: TYPE_NI_b8_b0 bug fix

Fix a decoding error where an 8-bit negative integer doesn't fit in an OPSZ_1
immed for TYPE_NI_b8_b0, found by testing on random inputs.

Review-URL: https://codereview.appspot.com/226230043

bl4ckic3/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,bl4ckic3/dynamorio,daksunt/dynamorio,AmesianX/dynamorio,code4bones/dynamorio,bl4ckic3/dynamorio,daksunt/dynamorio,code4bones/dynamorio,daksunt/dynamorio,code4bones/dynamorio,code4bones/dynamorio,bl4ckic3/dynamorio,bl4ckic3/dynamorio,AmesianX/dynamorio,code4bones/dynamorio,daksunt/dynamorio,daksunt/dynamorio,AmesianX/dynamorio