fix(renderer_markdown): Turn non-number indent_size to 1

Ref: #323

Author: OXY2DEV

lua/markview/renderers/markdown.lua

@@ -3457,7 +3457,7 @@ markdown.__list_item = function (buffer, item)
 	end
 
 	if type(indent_size) ~= "number" then
-		shift_width = 1;
+		indent_size = 1;
 	end
 
 	if item.marker == "-" then

lua/markview/spec.lua

@@ -769,6 +769,10 @@ spec.default = {
 			wrap = true,
 
 			indent_size = function (buffer)
+				if type(buffer) ~= "number" then
+					return vim.bo.shiftwidth or 4;
+				end
+
 				--- Use 'shiftwidth' value.
 				return vim.bo[buffer].shiftwidth or 4;
 			end,
@@ -2232,6 +2236,10 @@ spec.default = {
 			enable = true,
 
 			indent_size = function (buffer)
+				if type(buffer) ~= "number" then
+					return vim.bo.shiftwidth or 4;
+				end
+
 				--- Use 'shiftwidth' value.
 				return vim.bo[buffer].shiftwidth or 4;
 			end,