patch 8.1.0579: cannot attach properties to text

Problem:    Cannot attach properties to text.
Solution:   First part of adding text properties.
diff --git a/src/buffer.c b/src/buffer.c
index 8235a4f..bb94069 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -823,6 +823,9 @@
 #ifdef FEAT_SYN_HL
     syntax_clear(&buf->b_s);	    /* reset syntax info */
 #endif
+#ifdef FEAT_TEXT_PROP
+    clear_buf_prop_types(buf);
+#endif
     buf->b_flags &= ~BF_READERR;    /* a read error is no longer relevant */
 }