Initiate memtrace DPI only when trace_read_ready

This is required because otherwise we might overwrite into
the Verilog registers that contain a valid trace line that
was missed by downstream when it was not ready.  Basically
whenever trace_read_cycle stalls, we also want to stall
__in_* registers.

src/main/resources/vsrc/SimMemTrace.v

@@ -76,6 +76,10 @@ module SimMemTrace #(parameter FILENAME = "undefined", NUM_LANES = 4) (
       end
       __in_finished = 1'b0;
     end else begin
+      // We have to write to __in_ regs only when trace_read_ready, or
+      // otherwise we might overwrite lines that were previously valid
+      // but the downstream missed by being not ready.
+      if (trace_read_ready) begin
         for (integer tid = 0; tid < NUM_LANES; tid = tid + 1) begin
           memtrace_query(
             trace_read_ready,
@@ -94,4 +98,5 @@ module SimMemTrace #(parameter FILENAME = "undefined", NUM_LANES = 4) (
         end
       end
     end
+  end
 endmodule