Assert `sample` not null on entry to getNextImage

The aomCodecGetNextImage() and avmCodecGetNextImage() functions may set
`sample` to NULL internally.

src/codec_aom.c

diff --git a/src/codec_aom.c b/src/codec_aom.c
index cac507d..e2d0966 100644
--- a/src/codec_aom.c
+++ b/src/codec_aom.c
@@ -96,6 +96,8 @@
                                      avifBool * isLimitedRangeAlpha,
                                      avifImage * image)
 {
+    assert(sample);
+
     if (!codec->internal->decoderInitialized) {
         aom_codec_iface_t * const decoderInterface = aom_codec_av1_dx();
         struct aom_codec_stream_info streamInfo = { 0 };

src/codec_avm.c

diff --git a/src/codec_avm.c b/src/codec_avm.c
index e6aae65..d3432c0 100644
--- a/src/codec_avm.c
+++ b/src/codec_avm.c
@@ -59,6 +59,8 @@
                                      avifBool * isLimitedRangeAlpha,
                                      avifImage * image)
 {
+    assert(sample);
+
     if (!codec->internal->decoderInitialized) {
         AVIF_CHECKRES(avifCheckCodecVersionAVM());