liblzma: Remove an always-true condition from lzma_index_cat().

This should help static analysis tools to see that newg
isn't leaked.

Thanks to Pavel Raiskup.

src/liblzma/common/index.c

@@ -825,7 +825,7 @@ lzma_index_cat(lzma_index *restrict dest, lzma_index *restrict src,
 				s->groups.root = &newg->node;
 			}
 
-			if (s->groups.rightmost == &g->node)
+			assert(s->groups.rightmost == &g->node);
 			s->groups.rightmost = &newg->node;
 
 			lzma_free(g, allocator);