Set negative absolute SDF delays to zero.

The SDF standard allows negative absolute delays, but simulation
tools cannot handle this so set negative delays to zero.
(cherry picked from commit 301bbe94a0)

vpi/sys_sdf.c

@@ -194,6 +194,10 @@ void sdf_iopath_delays(int vpi_edge, const char*src, const char*dst,
 		  delay_vals[idx].type = vpiScaledRealTime;
 		  if (delval_list->val[idx].defined) {
 			delay_vals[idx].real = delval_list->val[idx].value;
+			  /* Simulation cannot support negative delays. */
+			if (delay_vals[idx].real < 0.0)  {
+			      delay_vals[idx].real = 0.0;
+			}
 		  }
  
 	    }