libFLAC/bitreader.c: Fix out-of-bounds read

Credit: Oss-Fuzz
Issue: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=17069
Testcase: fuzzer_decoder-5670265022840832

cherry-picked from upstream commit:
2e7931c27eb15e387da440a37f12437e35b22dd4

Bug: 156076070
Test: POC in bug description

Change-Id: Iafc2b895d75816177729f70feb53aec68e5c2aad
1 file changed