GUACAMOLE-2153: Fix GUAC_*_UTF16 on big-endian machine

Since UTF-16 works with 16-bit values it has a byte order and that byte
order is not well defined. There is a convention to use U+FEFF as first
character which allows determining the endianess but it's commonly not
present and doesn't seem to fit into the current code. The most common
thing is to assume little-endian which is what I did here and is already
implied by the test suite. If supporting both big- and little-endian
UTF-16 is wanted one could create _UTF16LE and _UTF16BE variations of
the functions.

Author: sertonix

src/common/iconv.c

@@ -22,6 +22,7 @@
 
 #include <guacamole/unicode.h>
 #include <stdint.h>
+#include <endian.h>
 
 /**
  * Lookup table for Unicode code points, indexed by CP-1252 codepoint.
@@ -109,7 +110,7 @@ int GUAC_READ_UTF16(const char** input, int remaining) {
         return 0;
 
     /* Read two bytes as integer */
-    value = *((uint16_t*) *input);
+    value = le16toh(*((uint16_t*) *input));
     *input += 2;
 
     return value;
@@ -213,7 +214,7 @@ void GUAC_WRITE_UTF16(char** output, int remaining, int value) {
         return;
 
     /* Write two bytes as integer */
-    *((uint16_t*) *output) = value;
+    *((uint16_t*) *output) = htole16(value);
     *output += 2;
 
 }