avutil/wchar_filename: Correct sizeof

Fixes: CID1591930 Wrong sizeof argument

Sponsored-by: Sovereign Tech Fund
Reviewed-by: Steve Lhomme <robux4@ycbcr.xyz>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e9e8bea2e7)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavutil/wchar_filename.h

@@ -57,7 +57,7 @@ static inline int wchartocp(unsigned int code_page, const wchar_t *filename_w,
         errno = EINVAL;
         return -1;
     }
-    *filename = (char*)av_malloc_array(num_chars, sizeof *filename);
+    *filename = av_malloc_array(num_chars, sizeof **filename);
     if (!*filename) {
         errno = ENOMEM;
         return -1;