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libtags: flac: check sample rate before dividing by zero; also check number of channels

--- a/sys/src/cmd/audio/libtags/flac.c

+++ b/sys/src/cmd/audio/libtags/flac.c

@@ -1,7 +1,7 @@

 /* https://xiph.org/flac/format.html */

 #include "tagspriv.h"

-#define beu3(d)   ((d)[0]<<16 | (d)[1]<<8  | (d)[2]<<0)

+#define beu3(d) ((d)[0]<<16 | (d)[1]<<8 | (d)[2]<<0)

 int

 tagflac(Tagctx *ctx)

@@ -18,6 +18,9 @@

 	sz = beu3(&d[5]); /* size of the stream info */

 	ctx->samplerate = beu3(&d[18]) >> 4;

 	ctx->channels = ((d[20]>>1) & 7) + 1;

+	if(ctx->samplerate < 1 || ctx->channels < 1)

+		return -1;

+

 	g = (uvlong)(d[21] & 0xf)<<32 | beu3(&d[22])<<8 | d[25];

 	ctx->duration = g * 1000 / ctx->samplerate;