lavu/video_enc_params: Avoid relying on an undefined C construct

The construct of using offsetof on a (potentially anonymous) struct
defined within the offsetof expression, while supported by all
current compilers, has been declared explicitly undefined by the
C standards committee [1].

Clang recently got a change to identify this as an issue [2];
initially it was treated as a hard error, but it was soon after
softened into a warning under the -Wgnu-offsetof-extensions option
(not enabled automatically as part of -Wall though).

Nevertheless - in this particular case, it's trivial to fix the
code not to rely on the construct that the standards committee has
explicitly called out as undefined.

[1] https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2350.htm
[2] https://reviews.llvm.org/D133574

Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/video_enc_params.c

@@ -27,11 +27,11 @@
 AVVideoEncParams *av_video_enc_params_alloc(enum AVVideoEncParamsType type,
                                             unsigned int nb_blocks, size_t *out_size)
 {
-    const size_t blocks_offset = offsetof(
+    struct TestStruct {
-        struct {
         AVVideoEncParams   p;
         AVVideoBlockParams b;
-        }, b);
+    };
+    const size_t blocks_offset = offsetof(struct TestStruct, b);
     size_t size = blocks_offset;
     AVVideoEncParams *par;