patch 9.0.0172: trying to allocate zero bytes

Problem:    Trying to allocate zero bytes.
Solution:   Do not allocate the proptype array when there are none.
            (closes #10867)
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index 9fae807..2a0ffc0 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -71,6 +71,18 @@
   call assert_fails("call prop_type_add('one', {'bufnr': 98764})", "E158:")
 endfunc
 
+def Test_proptype_add_remove()
+  # add and remove a prop type so that the array is empty
+  prop_type_add('local', {bufnr: bufnr('%')})
+  prop_type_delete('local', {bufnr: bufnr('%')})
+  prop_type_add('global', {highlight: 'ErrorMsg'})
+  prop_add(1, 1, {length: 1, type: 'global'})
+  redraw
+
+  prop_clear(1)
+  prop_type_delete('global')
+enddef
+
 def Test_proptype_buf_list()
   new
   var bufnr = bufnr('')
diff --git a/src/textprop.c b/src/textprop.c
index d5a53d6..45023e8 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -736,7 +736,7 @@
     int low = 0;
     int high;
 
-    if (ht == NULL)
+    if (ht == NULL || ht->ht_used == 0)
 	return NULL;
 
     // Make the loopup faster by creating an array with pointers to
diff --git a/src/version.c b/src/version.c
index 63a0eae..dec5fbd 100644
--- a/src/version.c
+++ b/src/version.c
@@ -736,6 +736,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    172,
+/**/
     171,
 /**/
     170,