patch 8.0.0571: negative line number when using :z^ in an empty buffer

Problem:    The cursor line number becomes negative when using :z^ in an empty
            buffer. (neovim #6557)
Solution:   Correct the line number.  Also reset the column.

src/ex_cmds.c

@@ -4664,6 +4664,8 @@ ex_z(exarg_T *eap)
 
     if (curs > curbuf->b_ml.ml_line_count)
 	curs = curbuf->b_ml.ml_line_count;
+    else if (curs < 1)
+	curs = 1;
 
     for (i = start; i <= end; i++)
     {
@@ -4686,7 +4688,11 @@ ex_z(exarg_T *eap)
 	}
     }
 
+    if (curwin->w_cursor.lnum != curs)
+    {
 	curwin->w_cursor.lnum = curs;
+	curwin->w_cursor.col = 0;
+    }
     ex_no_reprint = TRUE;
 }
 

src/testdir/test_ex_z.vim

@@ -68,7 +68,7 @@ func Test_z()
   bw!
 endfunc
 
-func Test_z_bug()
+func Test_z_overflow()
   " This used to access invalid memory as a result of an integer overflow
   " and freeze vim.
   normal ox
@@ -76,3 +76,10 @@ func Test_z_bug()
   z777777776666666
   ')
 endfunc
+
+func Test_z_negative_lnum()
+  new
+  z^
+  call assert_equal(1, line('.'))
+  bwipe!
+endfunc

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    571,
 /**/
     570,
 /**/