stream: fix endian swapping

In addition to the messed-up expression, the endianness was also
inverted. The code reads big endian by default.

It "worked" by coincidence, but for little endian, codepoints outside of
latin1 were broken.

The broken expression was found by Coverity.

stream/stream.c

@@ -861,8 +861,8 @@ static uint16_t stream_read_word_endian(stream_t *s, bool big_endian)
 {
     unsigned int y = stream_read_char(s);
     y = (y << 8) | stream_read_char(s);
-    if (big_endian)
-        y = (y >> 8) | ((y << 8) & 0xFF);
+    if (!big_endian)
+        y = ((y >> 8) & 0xFF) | (y << 8);
     return y;
 }