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sox-fmt: validate comments_bytes before use (CVE-2019-13590) [bug #325]

Cap the comments size to 1 GB to avoid overflows in subsequent
arithmetic.

The missing null check mentioned in the bug report is bogus since
lsx_calloc() returns a valid pointer or aborts.

--- a/src/sox-fmt.c

+++ b/src/sox-fmt.c

@@ -46,7 +46,9 @@

       lsx_readdw(ft, &comments_bytes))

     return SOX_EOF;

-  if (((headers_bytes + 4) & 7) || headers_bytes < FIXED_HDR + comments_bytes ||

+  if (((headers_bytes + 4) & 7) ||

+      comments_bytes > 0x40000000 || /* max 1 GB */

+      headers_bytes < FIXED_HDR + comments_bytes ||

       (num_channels > 65535)) /* Reserve top 16 bits */ {

     lsx_fail_errno(ft, SOX_EHDR, "invalid sox file format header");

     return SOX_EOF;