patch 8.1.0579: cannot attach properties to text

Problem:    Cannot attach properties to text.
Solution:   First part of adding text properties.

src/buffer.c

@@ -822,6 +822,9 @@ buf_freeall(buf_T *buf, int flags)
     }
 #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 */
 }