cli/crypto: fix segfault on failed gmime2 crypto context creation

Commit 1fdc08d0ff ("cli/crypto: treat failure to create a crypto
context as fatal.") started treating crypto context creation failures
"as fatal", returning NULL from _mime_node_create().

Unfortunately, we do not have NULL checks for _mime_node_create()
failures. The only caller, mime_node_child(), could check and return
NULL (as it's documented to do on errors) but none of the several call
sites have NULL checks either. And none of them really have a trivial
but feasible and graceful way of recovery.

So while the right thing to do would be to handle NULL returns
properly all over the place, and we have other scenarios that do
return NULL from above mentioned functions, the crypto context
creation failure is something that does seem to show up regularly in
some scenarios, revert back to the functionality before commit
1fdc08d0ff as an interim fix.

mime-node.c

@@ -247,7 +247,7 @@ _mime_node_create (mime_node_t *parent, GMimeObject *part)
 	const char *protocol = g_mime_content_type_get_parameter (content_type, "protocol");
 	cryptoctx = notmuch_crypto_get_context (node->ctx->crypto, protocol);
 	if (!cryptoctx)
-	    return NULL;
+	    return node;
     }
 #endif