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libtags: m4a: do not div by zero

--- a/sys/src/cmd/audio/libtags/m4a.c

+++ b/sys/src/cmd/audio/libtags/m4a.c

@@ -8,6 +8,7 @@

 tagm4a(Tagctx *ctx)

 {

 	uvlong duration;

+	uint x;

 	uchar *d;

 	int sz, type, dtype, i, skip, n;

@@ -101,12 +102,14 @@

 				if(ctx->read(ctx, d, 16) != 16)

 					return -1;

 				sz -= 16;

-				duration = beuint(&d[12]) / beuint(&d[8]);

+				if((x = beuint(&d[8])) > 0)

+					duration = beuint(&d[12]) / x;

 			}else if(d[1] == 1){ /* version 1 */

 				if(ctx->read(ctx, d, 28) != 28)

 					return -1;

 				sz -= 28;

-				duration = ((uvlong)beuint(&d[20])<<32 | beuint(&d[24])) / (uvlong)beuint(&d[16]);

+				if((x = beuint(&d[16])) > 0)

+					duration = ((uvlong)beuint(&d[20])<<32 | beuint(&d[24])) / (uvlong)x;

 			}

 			ctx->duration = duration * 1000;

 			continue;