Add sc_allocarray() util

Add a function to allocate an array, which fails safely in the case
where the multiplication would overflow.

app/src/util/memory.h

@@ -0,0 +1,12 @@
+#ifndef SC_MEMORY_H
+#define SC_MEMORY_H
+
+#include <stddef.h>
+
+/* Like calloc(), but without initialization.
+ * Like reallocarray(), but without reallocation.
+ */
+void *
+sc_allocarray(size_t nmemb, size_t size);
+
+#endif