FPE in generate_scaling() src/film_grain_tmpl.c

Reproduced with commit 12337860

Steps to reproduce:

replay testcase with ./dav1d_fuzzer testcase.ivf

==15569==ERROR: AddressSanitizer: FPE on unknown address 0x00000072821b (pc 0x00000072821b bp 0x7ffdf8e7ec30 sp 0x7ffdf8e7e8c0 T0)
    #0 0x72821a in generate_scaling src/film_grain_tmpl.c:185:55
    #1 0x72821a in dav1d_apply_grain_10bpc src/film_grain_tmpl.c:494
    #2 0x52d6c8 in output_image src/lib.c:202:9
    #3 0x52d21d in dav1d_get_picture src/lib.c
    #4 0x526630 in LLVMFuzzerTestOneInput tests/libfuzzer/dav1d_fuzzer.c:117:19
    #5 0x526d7f in main tests/libfuzzer/main.c:116:13
    #6 0x7f863c7fbb96 in __libc_start_main /build/glibc-OTsEL5/glibc-2.27/csu/../csu/libc-start.c:310
    #7 0x41aa19 in _start (dav1d_fuzzer+0x41aa19)

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