patch 9.0.0949: crash when unletting a variable while listing variables

Problem:    Crash when unletting a variable while listing variables.
Solution:   Disallow changing a hashtable while going over the entries.
            (closes #11435)
diff --git a/src/textprop.c b/src/textprop.c
index dee35c3..789ab8f 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -1789,7 +1789,7 @@
 	    }
 	    hash_init(*htp);
 	}
-	hash_add(*htp, PT2HIKEY(prop));
+	hash_add(*htp, PT2HIKEY(prop), "prop type");
     }
     else
     {
@@ -1924,7 +1924,7 @@
 	    ht = buf->b_proptypes;
 	    VIM_CLEAR(buf->b_proparray);
 	}
-	hash_remove(ht, hi);
+	hash_remove(ht, hi, "prop type delete");
 	vim_free(prop);
     }
 }