FPGA: Fix incorrect assignment in the db sample (#2329)

The state variable updates in MergeJoin and DuplicateMergeJoin involve a comparison of values taken from non-blocking pipe reads. These values need to be validated as coming from successful pipe reads before the state is updated.

Author: rho180

DirectProgramming/C++SYCL_FPGA/ReferenceDesigns/db/src/db_utils/MergeJoin.hpp

@@ -137,8 +137,11 @@ void MergeJoin() {
 
     //////////////////////////////////////////////////////
     //// state variables
-    move_t1_win = 
-      (t1_win.data.last().PrimaryKey() < t2_win.data.last().PrimaryKey());
+    // only update if both t1 and t2 windows come from valid pipe reads
+    if (t1_win_valid && t2_win_valid) {
+      move_t1_win =
+          (t1_win.data.last().PrimaryKey() < t2_win.data.last().PrimaryKey());
+    }
 
     keep_going = !t1_done || !t2_done;
     done_comp = t1_done || t2_done;
@@ -272,8 +275,11 @@ void DuplicateMergeJoin() {
 
     //////////////////////////////////////////////////////
     //// state variables
-    move_t1_win = 
-      (t1_win.data.last().PrimaryKey() < t2_win.data.last().PrimaryKey());
+    // only update if both t1 and t2 windows come from valid pipe reads
+    if (t1_win_valid && t2_win_valid) {
+      move_t1_win =
+          (t1_win.data.last().PrimaryKey() < t2_win.data.last().PrimaryKey());
+    }
 
     keep_going = !t1_done || !t2_done;
     done_comp = t1_done || t2_done;