engine: the Hanja search composes from its seed, and Escape gives it back

Ctrl+H took the pending syllable as the query, but the keys typed after
it started a composition of their own: gk, Ctrl+H, s showed 하ㄴ, and r,
Ctrl+H, k showed ㄱㅏ.  transstr now composes from a pending text, so
the search goes on where the syllable left off.  Escape ended the search
and dropped the syllable it had taken; it puts it back as pending text.

tests/vi_test.c

@@ -8,7 +8,7 @@ typevi(struct ct *t, char *keys, char *want)
 	Str out, raw;
 
 	raw = mkstr(keys);
-	transstr(&testvi, &raw, &out);
+	transstr(&testvi, nil, &raw, &out);
 	checkstr(t, keys, want, &out);
 }