mat4/mat5: fix int overflow in dataend calculation

The clang sanitizer warns of a possible signed integer overflow when
calculating the `dataend` value in `mat4_read_header()`.

```
src/mat4.c:323:41: runtime error: signed integer overflow: 205 * -100663296 cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior src/mat4.c:323:41 in
src/mat4.c:323:48: runtime error: signed integer overflow: 838860800 * 4 cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior src/mat4.c:323:48 in
```

Cast the offending `rows` and `cols` ints to `sf_count_t` (the type of
`dataend` before performing the calculation, to avoid the issue.

CVE: CVE-2022-33065
Fixes: libsndfile#789
Fixes: libsndfile#833

Signed-off-by: Alex Stewart <alex.stewart@ni.com>

src/mat4.c

@@ -320,7 +320,7 @@ mat4_read_header (SF_PRIVATE *psf)
 				psf->filelength - psf->dataoffset, psf->sf.channels * psf->sf.frames * psf->bytewidth) ;
 		}
 	else if ((psf->filelength - psf->dataoffset) > psf->sf.channels * psf->sf.frames * psf->bytewidth)
-		psf->dataend = psf->dataoffset + rows * cols * psf->bytewidth ;
+		psf->dataend = psf->dataoffset + (sf_count_t) rows * (sf_count_t) cols * psf->bytewidth ;
 
 	psf->datalength = psf->filelength - psf->dataoffset - psf->dataend ;