Replace strcpy with memcpy for alpha auxiliary URN

Replaces strcpy with memcpy when copying AVIF_URN_ALPHA0 into
alphaAuxProp->u.auxC.auxType in avifParseMinimizedImageBox().

AVIF_URN_ALPHA0 is 44 bytes and AUXTYPE_SIZE is 64, so there is no
overflow risk today. That said, every other string copy in this file
uses memcpy or avifROStreamReadString rather than strcpy, so this change
makes it consistent with the rest of the file.

src/read.c

diff --git a/src/read.c b/src/read.c
index 8592275..2842ad9 100644
--- a/src/read.c
+++ b/src/read.c
@@ -4525,7 +4525,7 @@
         avifProperty * alphaAuxProp = avifMetaCreateProperty(meta, "auxC");
         AVIF_CHECKERR(alphaAuxProp, AVIF_RESULT_OUT_OF_MEMORY);
         static_assert(sizeof(alphaAuxProp->u.auxC.auxType) >= sizeof(AVIF_URN_ALPHA0), "");
-        strcpy(alphaAuxProp->u.auxC.auxType, AVIF_URN_ALPHA0);
+        memcpy(alphaAuxProp->u.auxC.auxType, AVIF_URN_ALPHA0, sizeof(AVIF_URN_ALPHA0));
         AVIF_CHECKERR(avifDecoderItemAddProperty(alphaItem, alphaAuxProp), AVIF_RESULT_OUT_OF_MEMORY);
 
         // Property with fixed index 2 (reused).