borrow checker: reset index when empty, handle oom

crates/wiggle/src/error.rs

@@ -15,6 +15,8 @@ pub enum GuestError {
     PtrNotAligned(Region, u32),
     #[error("Pointer already borrowed: {0:?}")]
     PtrBorrowed(Region),
+    #[error("Borrow checker out of memory")]
+    BorrowCheckerOOM,
     #[error("Slice length mismatch")]
     SliceLengthsDiffer,
     #[error("In func {funcname}:{location}:")]