py/objarray: Fix array.append so it doesn't extend if append fails.

Addresses issue #1965.

py/objarray.c

@@ -336,7 +336,9 @@ STATIC mp_obj_t array_append(mp_obj_t self_in, mp_obj_t arg) {
         self->items = m_renew(byte, self->items, item_sz * self->len, item_sz * (self->len + self->free));
         mp_seq_clear(self->items, self->len + 1, self->len + self->free, item_sz);
     }
-    mp_binary_set_val_array(self->typecode, self->items, self->len++, arg);
+    mp_binary_set_val_array(self->typecode, self->items, self->len, arg);
+    // only update length/free if set succeeded
+    self->len++;
     self->free--;
     return mp_const_none; // return None, as per CPython
 }

tests/basics/bytearray_append.py

@@ -0,0 +1,15 @@
+# test bytearray.append method
+
+a = bytearray(4)
+print(a)
+
+# append should append a single byte
+a.append(2)
+print(a)
+
+# a should not be modified if append fails
+try:
+    a.append(None)
+except TypeError:
+    print('TypeError')
+print(a)