Fix compile warnings at tutorial-4.c and tutorial-5.c.

warning: initializing 'const jbyte *' (aka 'const signed char *') with an expression of type 'const char *' converts between pointers to integer types with different sign [-Wpointer-sign]
  const jbyte *char_uri = (*env)->GetStringUTFChars (env, uri, NULL);
               ^          ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

warning: passing 'const jbyte *' (aka 'const signed char *') to parameter of type 'const char *' converts between pointers to integer types with different sign [-Wpointer-sign]
  (*env)->ReleaseStringUTFChars (env, uri, char_uri);