Check OOM in avifImageCreate, avifEncoderCreate

The goal is to remove the abort() call from avifAlloc() once all
libavif functions check potential out-of-memory errors.
Add avifimagetest.
diff --git a/examples/avif_example_encode.c b/examples/avif_example_encode.c
index 0480ba4..31b00d1 100644
--- a/examples/avif_example_encode.c
+++ b/examples/avif_example_encode.c
@@ -26,6 +26,10 @@
     memset(&rgb, 0, sizeof(rgb));
 
     avifImage * image = avifImageCreate(128, 128, 8, AVIF_PIXEL_FORMAT_YUV444); // these values dictate what goes into the final AVIF
+    if (!image) {
+        fprintf(stderr, "Out of memory\n");
+        goto cleanup;
+    }
     // Configure image here: (see avif/avif.h)
     // * colorPrimaries
     // * transferCharacteristics
@@ -75,6 +79,10 @@
     }
 
     encoder = avifEncoderCreate();
+    if (!encoder) {
+        fprintf(stderr, "Out of memory\n");
+        goto cleanup;
+    }
     // Configure your encoder here (see avif/avif.h):
     // * maxThreads
     // * quality