patch 8.0.1443: compiler complains about uninitialized variable

Problem:    Compiler complains about uninitialized variable. (Tony Mechelynck)
Solution:   Assign a value to the variable.
diff --git a/src/undo.c b/src/undo.c
index d2ac457..0297e3f 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -2364,7 +2364,10 @@
 
     /* When "target" is 0; Back to origin. */
     if (target == 0)
-	goto found;
+    {
+	mark = lastmark;  /* avoid that GCC complains */
+	goto target_zero;
+    }
 
     /*
      * May do this twice:
@@ -2491,7 +2494,7 @@
 	    above = TRUE;	/* stop above the header */
     }
 
-found:
+target_zero:
     /* If we found it: Follow the path to go to where we want to be. */
     if (uhp != NULL || target == 0)
     {