depmod: Properly check index keys

[stoeckmann]

Synchronize character checks with libkmod-index.c. 8-bit ASCII chars would turn negative (due to signed char), which index__checkstring does not properly check for.

Fixes segmentation fault observed in https://github.com/kmod-project/kmod/issues/26

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files with missing lines	Coverage Δ
tools/depmod.c	57.52% <100.00%> (ø)

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[lucasdemarchi]

Applied, thanks