Fix incremental receive silently failing for recursive sends

The problem occurs because dmu_recv_begin pulls in the payload and 
next header from the input stream in order to use the contents of 
the begin record's nvlist. However, the change to do that before the 
other checks in dmu_recv_begin occur caused a regression where an 
empty send stream in a recursive send could have its END record 
consumed by this, which broke the logic of recv_skip. A test is 
also included to protect against this case in the future.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #12661
Closes #14568

tests/zfs-tests/cmd/libzfs_input_check.c

@@ -563,7 +563,7 @@ test_recv_new(const char *dataset, int fd)
 	fnvlist_add_uint64(optional, "action_handle", *action_handle);
 #endif
 	IOC_INPUT_TEST(ZFS_IOC_RECV_NEW, dataset, required, optional,
-	    ZFS_ERR_STREAM_TRUNCATED);
+	    ENOTSUP);
 
 	nvlist_free(props);
 	nvlist_free(optional);