Fix array allocation when slot size < 40 bytes

We need to allocate at least sizeof(struct RArray) when the array is
embedded on garbage collectors that support slot sizes less than 40 bytes.

Author: peterzhu2118

array.c

@@ -194,7 +194,9 @@ ary_embed_capa(VALUE ary)
 static size_t
 ary_embed_size(long capa)
 {
-    return offsetof(struct RArray, as.ary) + (sizeof(VALUE) * capa);
+    size_t size = offsetof(struct RArray, as.ary) + (sizeof(VALUE) * capa);
+    if (size < sizeof(struct RArray)) size = sizeof(struct RArray);
+    return size;
 }
 
 static bool