dmg_fp: Fix a char type array subscript

Summary:
Fixes the following:
external/chromium_org/base/third_party/dmg_fp/dtoa.cc:1562:13: error: array subscript is of type 'char'
[-Werror,-Wchar-subscripts]
        if (!hexdig['0'])
                   ^~~~

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D142