efikeygen: Account for the signature size in bundle_signature()

In ea7a2c4, when bundling the signature, the bitstring type field
is being set manually with a hacky offset.  That offset is only valid
with specific signature types, and so with any signature of a different
size, this is just corrupting data either in the signature or after it.

This change from Egor fixes the egregious hack to manually set the type
so that it computes the location based on the signature length, rather
than hard-coding a value.

Signed-off-by: Peter Jones <pjones@redhat.com>

src/efikeygen.c

@@ -141,7 +141,8 @@ bundle_signature(cms_context *cms, SECItem *sigder, SECItem *data,
 		errx(1, "could not encode certificate: %s",
 			PORT_ErrorToString(PORT_GetError()));
 
-	sigder->data[sigder->len - 261] = DER_BIT_STRING;
+	//Note: offset is signature size + 5 bytes for DER encoding
+	sigder->data[sigder->len - (signature->len + 5)] = DER_BIT_STRING;
 
 	return 0;
 }