add missing bit to UTF-16 surrogate pair conversion

src/physfs_unicode.c

@@ -210,7 +210,7 @@ static PHYSFS_uint32 utf16codepoint(const PHYSFS_uint16 **_str)
         else
         {
             src++;  /* eat the other surrogate. */
-            cp = (((cp - 0xD800) << 10) | (pair - 0xDC00));
+            cp = (0x10000 | ((cp - 0xD800) << 10) | (pair - 0xDC00));
         } /* else */
     } /* else if */