avformat/utils: avoid void pointer arithmetic

Fixes compliation on MSVC.

Regression since cb708d8703.

Based on code by James Almer.

Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/hlsenc.c

@@ -1099,7 +1099,7 @@ static int hls_append_segment(struct AVFormatContext *s, HLSContext *hls,
         av_bprint_chars(&bp, 0, 1);
     }
 
-    en = ff_bprint_finalize_as_fam(&bp, &en0, sizeof(en0), en0.buf);
+    en = ff_bprint_finalize_as_fam(&bp, &en0, offsetof(HLSSegment, buf));
     if (!en)
         return AVERROR(ENOMEM);
 #define NEXT(s) ((s) + strlen(s) + 1)

libavformat/internal.h

@@ -706,11 +706,10 @@ int ff_make_codec_str(void *logctx, const AVCodecParameters *par,
  *
  * @param bp pointer to an AVBprint struct
  * @param struct_ptr pointer to the struct to be copied
- * @param struct_size must be sizeof(*struct_ptr)
- * @param flex_member must be struct_ptr->flexible_array_member
+ * @param fam_offset must be offsetof(StructType, flexible_array_member)
  * @return pointer to the newly allocated struct, NULL on allocation error or
  *         if the AVBPrint buffer is not complete
  */
-void *ff_bprint_finalize_as_fam(struct AVBPrint *bp, const void *struct_ptr, size_t struct_size, void *flex_member);
+void *ff_bprint_finalize_as_fam(struct AVBPrint *bp, const void *struct_ptr, size_t fam_offset);
 
 #endif /* AVFORMAT_INTERNAL_H */

libavformat/utils.c

@@ -685,20 +685,20 @@ int ff_parse_opts_from_query_string(void *obj, const char *str, int allow_unknow
     return 0;
 }
 
-void *ff_bprint_finalize_as_fam(struct AVBPrint *bp, const void *struct_ptr, size_t struct_size, void *flex_member)
+void *ff_bprint_finalize_as_fam(struct AVBPrint *bp, const void *struct_ptr, size_t fam_offset)
 {
     if (!av_bprint_is_complete(bp)) {
         av_bprint_finalize(bp, NULL);
         return NULL;
     }
 
-    void *p = av_malloc(struct_size + bp->len);
+    uint8_t *p = av_malloc(fam_offset + bp->len);
     if (!p) {
         av_bprint_finalize(bp, NULL);
         return NULL;
     }
-    memcpy(p, struct_ptr, struct_size);
-    memcpy(p + (flex_member - struct_ptr), bp->str, bp->len);
+    memcpy(p, struct_ptr, fam_offset);
+    memcpy(p + fam_offset, bp->str, bp->len);
     av_bprint_finalize(bp, NULL);
 
     return p;