xml.c: fix typo / regression in checking codepoint range for utf-16 surrogate pair

Regression in commit 12b279581fbbcde2b36eb4b78d70a1c52d4a209a

0xdffff should be 0xdfff.

printf '<item><title>&#x1f448;</title></item>' | sfeed

Before (bad):
	&#x1f448;

After:
	👈

1 files changed, 1 insertions, 1 deletions

diff --git a/xml.c b/xml.c
index 8d8017f..ae106f2 100644
--- a/xml.c
+++ b/xml.c
@@ -253,7 +253,7 @@ numericentitytostr(const char *e, char *buf, size_t bufsiz)
 		l = strtol(e, &end, 10);
 	/* invalid value or not a well-formed entity or invalid code point */
 	if (errno || e == end || *end != ';' || l < 0 || l > 0x10ffff ||
-	    (l >= 0xd800 && l <= 0xdffff))
+	    (l >= 0xd800 && l <= 0xdfff))
 		return -1;
 	len = codepointtoutf8(l, buf);
 	buf[len] = '\0';