Move an assertion next to the dependent statement

In avifCodecDecodeInputFillFromDecoderItem(), the assertion

        assert(lselProp->u.lsel.layerID < MAX_AV1_LAYER_COUNT);

is apparently intended to ensure that the following uint8_t cast is
safe:

        sample->spatialID = (uint8_t)lselProp->u.lsel.layerID;

So move the assertion next to that statement.

src/read.c

diff --git a/src/read.c b/src/read.c
index c77944c..6c3368b 100644
--- a/src/read.c
+++ b/src/read.c
@@ -549,9 +549,9 @@
 
         avifDecodeSample * sample = (avifDecodeSample *)avifArrayPushPtr(&decodeInput->samples);
         sample->itemID = item->id;
-        assert(lselProp->u.lsel.layerID < MAX_AV1_LAYER_COUNT);
         sample->offset = 0;
         sample->size = sampleSize;
+        assert(lselProp->u.lsel.layerID < MAX_AV1_LAYER_COUNT);
         sample->spatialID = (uint8_t)lselProp->u.lsel.layerID;
         sample->sync = AVIF_TRUE;
     } else if (allowProgressive && item->progressive) {