fix(utils): don't const_allocate(0, _)

I noticed that the test code from [the PR constifying `Box`'s methods]
[1] avoids calling `const_allocate` with size zero.

[1]: https://github.com/rust-lang/rust/pull/91884/files#diff-5ebca97f446e7823021939db776e4dca5563ec3409e55295387553b279b51521R66

Author: yvt

src/r3_core/src/utils/alloc/allocator.rs

@@ -394,6 +394,8 @@ unsafe impl const rlsf::FlexSource for ConstFlexSource {
             return None;
         };
 
+        assert!(min_size != 0);
+
         // FIXME: Directly calling `const_allocate` from here causes the
         //        compiler to panic
         // Safety: `const_allocate_{in_const, at_rt}` behave observably

src/r3_core/src/utils/alloc/freeze.rs

@@ -47,6 +47,11 @@ impl<T: Copy> Frozen<T> {
             .checked_mul(x.len())
             .expect("size overflow");
         let align = core::mem::align_of::<T>();
+
+        if size == 0 {
+            return &[];
+        }
+
         unsafe {
             // Allocate a CTFE heap memory block
             let ptr = core::intrinsics::const_allocate(size, align).cast::<T>();