iconv was not returning -1 on most failure

this broke most uses of iconv in real-world programs, especially
glib's iconv wrappers.

src/locale/iconv.c

@@ -559,9 +559,11 @@ ilseq:
 	goto end;
 toobig:
 	err = E2BIG;
+	x = -1;
 	goto end;
 starved:
 	err = EINVAL;
+	x = -1;
 end:
 	errno = err;
 	return x;