decode_font: fix subtraction broken by change to unsigned type

This caused a one-byte buffer overwrite and an assertion failure.

Regression in commit 910211f1c0078e37546f73e95306724358b89be2.

Discovered by OSS-Fuzz.

Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=26674.
Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=26678.