avcodec/amrwbdec: Fix division by 0 in voice_factor()

The added value matches "Digital cellular telecommunications system (Phase 2+) (GSM); Universal Mobile Telecommunications System (UMTS); LTE; Extended Adaptive Multi-Rate - Wideband (AMR-WB+) codec; Floating-point ANSI-C code (3GPP TS 26.304 version 14.0.0 Release 14)
Extended Adaptive Multi-Rate - Wideband (AMR-WB+) codec; Floating-point ANSI-C code"

Fixes: runtime error: division by zero
Fixes: 4415/clusterfuzz-testcase-minimized-4677752314658816

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
