Address review comments

Author: oli-obk

src/shims/foreign_items.rs

@@ -272,16 +272,13 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
                 )?;
             }
             "__rust_realloc" => {
-                let ptr = match this.read_scalar(args[0])?.not_undef()? {
-                    Scalar::Ptr(p) => p,
-                    Scalar::Raw { .. } => throw_ub_format!("cannot reallocate integer addresses"),
-                };
                 let old_size = this.read_scalar(args[1])?.to_machine_usize(this)?;
                 let align = this.read_scalar(args[2])?.to_machine_usize(this)?;
                 let new_size = this.read_scalar(args[3])?.to_machine_usize(this)?;
                 if old_size == 0 || new_size == 0 {
                     throw_unsup!(HeapAllocZeroBytes);
                 }
+                let ptr = this.force_ptr(this.read_scalar(args[0])?)?;
                 if !align.is_power_of_two() {
                     throw_unsup!(HeapAllocNonPowerOfTwoAlignment(align));
                 }