base: Fix the type of variable |a| in MD5DigestToBase16().
base: Fix the type of variable |a| in MD5DigestToBase16().

The type of MD5Digest's |a| member is uint8_t, which is in turn unsigned
char, not int.

BUG=None
TEST=base_unittests --gtest_filter=MD5*
R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1078403003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#324976}

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