patch 8.1.1387: calling prop_add() in an empty buffer doesn't work

Problem:    Calling prop_add() in an empty buffer doesn't work. (Dominique
            Pelle)
Solution:   Open the memline before adding a text property. (closes #4412)

src/testdir/test_textprop.vim

@@ -752,3 +752,11 @@ func Test_textprop_screenshot_visual()
   " Same, but delete four columns
   call RunTestVisualBlock(4, '02')
 endfunc
+
+" Adding a text property to a new buffer should not fail
+func Test_textprop_empty_buffer()
+  call prop_type_add('comment', {'highlight': 'Search'})
+  new
+  call prop_add(1, 1, {'type': 'comment'})
+  close
+endfunc

src/textprop.c

@@ -12,6 +12,7 @@
  *
  * TODO:
  * - Adjust text property column and length when text is inserted/deleted.
+ *   -> :substitute with multiple matches, issue #4427
  *   -> a :substitute with a multi-line match
  *   -> search for changed_bytes() from misc1.c
  *   -> search for mark_col_adjust()
@@ -238,6 +239,9 @@ f_prop_add(typval_T *argvars, typval_T *rettv UNUSED)
 	return;
     }
 
+    if (buf->b_ml.ml_mfp == NULL)
+	ml_open(buf);
+
     for (lnum = start_lnum; lnum <= end_lnum; ++lnum)
     {
 	colnr_T col;	// start column
@@ -327,7 +331,7 @@ get_text_props(buf_T *buf, linenr_T lnum, char_u **props, int will_change)
 
     // Be quick when no text property types have been defined or the buffer,
     // unless we are adding one.
-    if (!buf->b_has_textprop && !will_change)
+    if ((!buf->b_has_textprop && !will_change) || buf->b_ml.ml_mfp == NULL)
 	return 0;
 
     // Fetch the line to get the ml_line_len field updated.

src/version.c

@@ -767,6 +767,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1387,
 /**/
     1386,
 /**/