std: Add a test for allocations requesting at least N bytes

lib/std/heap.zig

@@ -919,6 +919,13 @@ pub fn testAllocator(base_allocator: *mem.Allocator) !void {
     const zero_bit_ptr = try allocator.create(u0);
     zero_bit_ptr.* = 0;
     allocator.destroy(zero_bit_ptr);
+
+    const oversize = try allocator.allocAdvanced(u32, null, 5, .at_least);
+    testing.expect(oversize.len >= 5);
+    for (oversize) |*item| {
+        item.* = 0xDEADBEEF;
+    }
+    allocator.free(oversize);
 }
 
 pub fn testAllocatorAligned(base_allocator: *mem.Allocator, comptime alignment: u29) !void {

lib/std/heap/arena_allocator.zig

@@ -82,16 +82,17 @@ pub const ArenaAllocator = struct {
                 return result;
             }
 
+            const bigger_buf_size = @sizeOf(BufNode) + new_end_index;
             // Try to grow the buffer in-place
             if (self.child_allocator.resizeFn(
                 self.child_allocator,
                 cur_node.data,
                 @alignOf(BufNode),
-                new_end_index,
+                bigger_buf_size,
                 1,
                 @returnAddress(),
             )) |new_size| {
-                assert(new_size >= new_end_index);
+                assert(new_size >= bigger_buf_size);
             } else |err| switch (err) {
                 error.OutOfMemory => {
                     // Allocate a new node if that's not possible