patch 9.0.0975: virtual text below empty line misplaced when 'number' set

Problem:    Virtual text below an empty line is misplaced when 'number' is
            set.
Solution:   Adjust the computations. (closes #11629)

src/drawline.c

@@ -663,9 +663,9 @@ text_prop_position(
 		before -= cells;
 
 	    // Below-align: empty line add one character
-	    if (below && vcol == 0 && col_with_padding == 0
-						      && wp->w_width == before)
-		col_with_padding = 1;
+	    if (below && vcol == 0 && col_with_padding == col_off
+					    && wp->w_width - col_off == before)
+		col_with_padding += 1;
 
 	    if (before < 0
 		    || !(right || below)

src/testdir/dumps/Test_prop_below_after_empty_2.dump

@@ -0,0 +1,8 @@
+| +0#af5f00255#ffffff0@1|1| >v+0#0000000&|i|m|9|s|c|r|i|p|t| @45
+| +0#af5f00255&@1|2| | +0#0000000&@55
+| +0#af5f00255&@3| +0#0000000&|T+0#ffffff16#e000002|h|e| |q|u|i|c|k| |b|r|o|w|n| |f|o|x| |j|u|m|p|s| |o|v|e|r| |t|h|e| |l|a|z|y| |d|o|g| +0#0000000#ffffff0@11
+| +0#af5f00255&@1|3| |t+0#0000000&|h|r|e@1| @50
+| +0#af5f00255&@1|4| | +0#0000000&@55
+| +0#af5f00255&@3|T+0#0000000#ffd7ff255|h|e| |s|l|o|w| |f|o|x| |b|u|m|p|s| |i|n|t|o| |t|h|e| |l|a|z|y| |d|o|g| +0&#ffffff0@19
+|~+0#4040ff13&| @58
+|:+0#0000000&|s|e|t| |n|u|m|b|e|r| @30|1|,|1| @10|A|l@1| 

src/testdir/test_textprop.vim

@@ -2744,6 +2744,9 @@ func Test_prop_with_text_below_after_empty()
   let buf = RunVimInTerminal('-S XscriptPropBelowAfterEmpty', #{rows: 8, cols: 60})
   call VerifyScreenDump(buf, 'Test_prop_below_after_empty_1', {}) 
 
+  call term_sendkeys(buf, ":set number\<CR>")
+  call VerifyScreenDump(buf, 'Test_prop_below_after_empty_2', {}) 
+
   call StopVimInTerminal(buf)
 endfunc
 

src/version.c

@@ -695,6 +695,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    975,
 /**/
     974,
 /**/