HPCC-26562 Avoid potential read beyond end of buffer in decodePTreeNameHexEncoding

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>

system/jlib/jptree.cpp

@@ -9805,7 +9805,7 @@ static void decodePTreeNameHexEncoding(StringBuffer &s, const char *&input, unsi
         remaining -= 2;
         s.append(ch);
     }
-    if (*input=='_')
+    if (remaining && (*input=='_'))
     {
         input++;
         remaining--;