Restrict metaflac fuzzer to offered file (#885) #424

Job	Run time
build (no-ogg, --disable-ogg)	2m 22s
build (integer-only-library, -DFLAC__INTEGER_ONLY_LIBRARY)	2m 30s
build (32-bit-words, --disable-64-bit-words)	2m 43s
build (no-asm, --disable-asm-optimizations)	2m 25s
	10m 0s

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