Fix overrun when comparing with unterminated ICU language string.

The overrun was introduced in commit c4ff35f10.

Author: Andreas Karlsson <andreas@proxel.se>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/96d80a47-f17f-42fa-82b1-2908efbd6541@gmail.com
Backpatch-through: 18

src/backend/utils/adt/pg_locale_icu.c

@@ -989,11 +989,11 @@ static int32_t
 foldcase_options(const char *locale)
 {
 	uint32		options = U_FOLD_CASE_DEFAULT;
-	char		lang[3];
+	char		lang[ULOC_LANG_CAPACITY];
 	UErrorCode	status = U_ZERO_ERROR;
 
-	uloc_getLanguage(locale, lang, 3, &status);
-	if (U_SUCCESS(status))
+	uloc_getLanguage(locale, lang, ULOC_LANG_CAPACITY, &status);
+	if (U_SUCCESS(status) && status != U_STRING_NOT_TERMINATED_WARNING)
 	{
 		/*
 		 * The option name is confusing, but it causes u_strFoldCase to use