lavc: add ff_bprint_to_extradata() helper and use it.

This commit also makes sure the extradata and subtitle_header are NUL
terminated, without taking into account the trailing '\0' in account in
the size.

At the same time, it should fix 'warning: dereferencing type-punned
pointer will break strict-aliasing rules' warning for compilers who
don't consider uint8_t** and char** compatibles.

libavcodec/utils.c

@@ -27,6 +27,7 @@
 
 #include "libavutil/avassert.h"
 #include "libavutil/avstring.h"
+#include "libavutil/bprint.h"
 #include "libavutil/channel_layout.h"
 #include "libavutil/crc.h"
 #include "libavutil/mathematics.h"
@@ -2684,3 +2685,21 @@ int avcodec_is_open(AVCodecContext *s)
 {
     return !!s->internal;
 }
+
+int ff_bprint_to_extradata(AVCodecContext *avctx, struct AVBPrint *buf)
+{
+    int ret;
+    char *str;
+
+    ret = av_bprint_finalize(buf, &str);
+    if (ret < 0)
+        return ret;
+    avctx->extradata = str;
+    /* Note: the string is NUL terminated (so extradata can be read as a
+     * string), but the ending character is not accounted in the size (in
+     * binary formats you are likely not supposed to mux that character). When
+     * extradata is copied, it is also padded with FF_INPUT_BUFFER_PADDING_SIZE
+     * zeros. */
+    avctx->extradata_size = buf->len;
+    return 0;
+}