Fix undefined behavior on memcpy()

libnetconf2's test `test_server_thread` eventually ends up in this stack
trace:

 libyang/src/tree_schema.c:3288:16: runtime error: null pointer passed as argument 1, which is declared to never be null
 /nix/store/ypih4394q488ljr421x8jak55vmr0ckn-glibc-2.32-dev/include/string.h:44:28: note: nonnull attribute specified here
    #0 0x7f565ebf8f6e in lys_node_dup_recursion libyang/src/tree_schema.c:3288:9
    CESNET#1 0x7f565ebf3409 in lys_node_dup libyang/src/tree_schema.c:3609:14
    CESNET#2 0x7f565ebbf62c in yang_check_deviation libyang/src/parser_yang.c:4646:26
    CESNET#3 0x7f565eba6531 in yang_check_sub_module libyang/src/parser_yang.c:4784:13
    CESNET#4 0x7f565eba3d1c in yang_read_module libyang/src/parser_yang.c:2705:13
    CESNET#5 0x7f565ebd1077 in lys_parse_mem_ libyang/src/tree_schema.c:1083:15
    CESNET#6 0x7f565ebd31ab in lys_parse_fd_ libyang/src/tree_schema.c:1271:14
    CESNET#7 0x7f565e92f291 in ly_ctx_load_localfile libyang/src/context.c:916:39
    CESNET#8 0x7f565e92c31b in ly_ctx_load_sub_module libyang/src/context.c:1063:19
    CESNET#9 0x7f565e92fdcc in ly_ctx_load_module libyang/src/context.c:1102:12
    CESNET#10 0x597422 in main libnetconf2/tests/test_server_thread.c:686:5
    CESNET#11 0x7f565dc98dbc in __libc_start_main (/nix/store/kah5n342wz4i0s9lz9ka4bgz91xa2i94-glibc-2.32/lib/libc.so.6+0x23dbc)
    CESNET#12 0x42a4d9 in _start /build/glibc-2.32/csu/../sysdeps/x86_64/start.S:120

In other words, it would call `memcpy` with the source parameter being
NULL, which is [explicitly said to be undefined
behavior](https://www.imperialviolet.org/2016/06/26/nonnull.html).

src/tree_schema.c

@@ -3285,7 +3285,9 @@ lys_node_dup_recursion(struct lys_module *module, struct lys_node *parent, const
             }
         }
     } else {
-        memcpy(retval->iffeature, node->iffeature, retval->iffeature_size * sizeof *retval->iffeature);
+        if (node->iffeature_size) {
+            memcpy(retval->iffeature, node->iffeature, retval->iffeature_size * sizeof *retval->iffeature);
+        }
     }
 
     /*