local+reference: fix tracking offset after failed step

If a clock step enabled by the makestep directive or requested by the
makestep command fails, accumulate the missing step back to keep the
tracking offset valid.

This fixes time served by an instance configured with the makestep
directive and the -x option (the null driver cannot perform steps) at
the same time. It will still generate error log messages.

local.c

@@ -695,8 +695,11 @@ LCL_MakeStep(void)
 
   /* Cancel remaining slew and make the step */
   LCL_AccumulateOffset(correction, 0.0);
-  if (!LCL_ApplyStepOffset(-correction))
+  if (!LCL_ApplyStepOffset(-correction)) {
+    /* Revert the correction */
+    LCL_AccumulateOffset(-correction, 0.0);
     return 0;
+  }
 
   LOG(LOGS_WARN, "System clock was stepped by %.6f seconds", correction);