Protect against small overread in SASLprep validation

In case of torn UTF8 in the input data we might end up going
past the end of the string since we don't account for length.
While validation won't be performed on a sequence with a NULL
byte it's better to avoid going past the end to beging with.
Fix by taking the length into consideration.

Author: Jacob Champion <jacob.champion@enterprisedb.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/CAOYmi+mTnmM172g=_+Yvc47hzzeAsYPy2C4UBY3HK9p-AXNV0g@mail.gmail.com

src/common/saslprep.c

@@ -1004,15 +1004,17 @@ pg_utf8_string_len(const char *source)
 	const unsigned char *p = (const unsigned char *) source;
 	int			l;
 	int			num_chars = 0;
+	size_t		len = strlen(source);
 
-	while (*p)
+	while (len)
 	{
 		l = pg_utf_mblen(p);
 
-		if (!pg_utf8_islegal(p, l))
+		if (len < l || !pg_utf8_islegal(p, l))
 			return -1;
 
 		p += l;
+		len -= l;
 		num_chars++;
 	}