Add sc_allocarray() util

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

app/meson.build

@@ -35,6 +35,7 @@ src = [
     'src/util/intmap.c',
     'src/util/intr.c',
     'src/util/log.c',
+    'src/util/memory.c',
     'src/util/net.c',
     'src/util/net_intr.c',
     'src/util/process.c',

app/src/util/memory.c

@@ -0,0 +1,14 @@
+#include "memory.h"
+
+#include <stdlib.h>
+#include <errno.h>
+
+void *
+sc_allocarray(size_t nmemb, size_t size) {
+    size_t bytes;
+    if (__builtin_mul_overflow(nmemb, size, &bytes)) {
+      errno = ENOMEM;
+      return NULL;
+    }
+    return malloc(bytes);
+}

app/src/util/memory.h

@@ -0,0 +1,15 @@
+#ifndef SC_MEMORY_H
+#define SC_MEMORY_H
+
+#include <stddef.h>
+
+/**
+ * Allocate an array of `nmemb` items of `size` bytes each
+ *
+ * Like calloc(), but without initialization.
+ * Like reallocarray(), but without reallocation.
+ */
+void *
+sc_allocarray(size_t nmemb, size_t size);
+
+#endif