Fix DERIVE_CVT3_FROM_DMT

Detail:
  c/ScrModes - Swap 'return;' for 'return NULL;' in the (currently unused) DERIVE_CVT3_FROM_DMT code to ensure the function has a sensible return value
Admin:
  Untested


Version 0.47. Retagged as 'ScrModes-0_47'

c/ScrModes

@@ -2315,7 +2315,7 @@ static _kernel_oserror *generate_cvt3_timing(char cvt1, char cvt2, char cvt3, Mo
             if (add_proposed_mode(monitor, mode_desc) == 0) {
               free(mode_desc);
             }
-            return;
+            return NULL;
         }
         i++;
     }