patch 8.0.0542: getpos() can return a negative line number

Problem:    getpos() can return a negative line number. (haya14busa)
Solution:   Handle a zero topline and botline. (closes #1613)

Author: brammool

runtime/doc/eval.txt

@@ -5413,8 +5413,10 @@ line({expr})	The result is a Number, which is the line number of the file
 		    $	    the last line in the current buffer
 		    'x	    position of mark x (if the mark is not set, 0 is
 			    returned)
-		    w0	    first line visible in current window
-		    w$	    last line visible in current window
+		    w0	    first line visible in current window (one if the
+			    display isn't updated, e.g. in silent Ex mode)
+		    w$	    last line visible in current window (this is one
+			    less than "w0" if no lines are visible)
 		    v	    In Visual mode: the start of the Visual area (the
 			    cursor is the end).  When not in Visual mode
 			    returns the cursor position.  Differs from |'<| in

src/eval.c

@@ -6120,13 +6120,16 @@ var2fpos(
 	if (name[1] == '0')		/* "w0": first visible line */
 	{
 	    update_topline();
-	    pos.lnum = curwin->w_topline;
+	    /* In silent Ex mode topline is zero, but that's not a valid line
+	     * number; use one instead. */
+	    pos.lnum = curwin->w_topline > 0 ? curwin->w_topline : 1;
 	    return &pos;
 	}
 	else if (name[1] == '$')	/* "w$": last visible line */
 	{
 	    validate_botline();
-	    pos.lnum = curwin->w_botline - 1;
+	    /* In silent Ex mode botline is zero, return zero then. */
+	    pos.lnum = curwin->w_botline > 0 ? curwin->w_botline - 1 : 0;
 	    return &pos;
 	}
     }

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    542,
 /**/
     541,
 /**/