Soft assert that the count in GuardSubquerySize is >=1

Author: ggevay

src/expr/src/relation/func.rs

@@ -3943,7 +3943,16 @@ impl TableFunc {
                 // We error if the count is not one,
                 // and produce no rows if equal to one.
                 let count = datums[0].unwrap_int64();
+                // Note that we can't get a 0 count here, since the `Reduce count` below us is not
+                // an SQL `count`, but an MIR `count`, which has an empty output on an empty input.
+                // If we get a negative count, then we have a negative accumulation error somewhere.
+                soft_assert_or_log!(
+                    count >= 1,
+                    "GuardSubquerySize encountered invalid count: {}",
+                    count
+                );
                 if count != 1 {
+                    // TODO(ggevay): Add count to error msg after the old subquery code is gone.
                     Err(EvalError::MultipleRowsFromSubquery)
                 } else {
                     Ok(Box::new([].into_iter()))