engine: nothing pending is no reading, and reaches into nothing

startsearch let reachback walk the client's text whenever Ctrl+H found
im.sel < 0, without asking whether anything was pending, so a Ctrl+H
pressed to begin a reading took the syllables already written instead.
With the cursor after 입니다 the query became 다, the preedit showed
nothing at all, and Space committed 多 over the 다 the user had written;
typing the reading the key was pressed for gave 다한, so 입니다漢 came
out 입니多恨.  A reading is what is pending, and there is none.

The seeded reach is untouched: 한 committed with 자 pending still
converts 한자 as a word and takes the 한 back.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

strans.c

@@ -793,7 +793,9 @@ startsearch(int lang, Str *com)
 	sclear(&seed);
 	if(lang == LangHANJA && !search.lang && im.sel < 0){
 		impre(&im, &seed);
-		reachback(&seed, &back);
+		/* Nothing pending is no reading, and reaches into nothing. */
+		if(seed.n != 0)
+			reachback(&seed, &back);
 	}else
 		flush(com);
 	reset();

tests/engine_test.c

@@ -2005,6 +2005,22 @@ engine_hanja_reaches_back(struct ct *t)
 		CT_EQ_INT(t, 0, takeback);
 	}
 
+	/* Nothing pending is no reading, and reaches into nothing. */
+	surround = mkstr("민");
+	sclear(&im.pre);
+	sclear(&com);
+	CT_CHECK(t, keystroke('h', Mctrl, &com));
+	CT_EQ_INT(t, 0, search.back.n);
+	CT_EQ_INT(t, 0, im.nkouho);
+	if(typekeys(t, "als", &com)){
+		checkstr(t, "the reading typed is the whole query", "민",
+			&search.text);
+		CT_CHECK(t, keystroke(Kret, 0, &com));
+		checkstr(t, "the client's syllable is not converted", "民",
+			&com);
+		CT_EQ_INT(t, 0, takeback);
+	}
+
 	/* The frontend's own request carries the text and is told to take it. */
 	init();
 	im.l = getlang(LangKO);