Avoid trying to throw multiple exceptions at once.

The typical usage pattern for the get_char helper function is:

    bool thrown = false;

    n = get_char(env, s, 0, 1000, &thrown);
    n += get_char(env, s, 1, 100, &thrown);
    n += get_char(env, s, 2, 10, &thrown);
    n += get_char(env, s, 3, 1, &thrown);
    if (thrown) return false;

As you can see, get_char is called multiple times before the
thrown flag is checked. If the input text contains multiple
incorrect characters, then we have to guard against throwing
the same exception multiple times. (Because doing so will
cause the Dalvik runtime to abort.)

The fix is simple: modify get_char to check if an exception
has already been thrown before throwing a new exception.