fix misusing of fread in example

[Saafo]

The use of fread function in example code is:

libopusenc/examples/opusenc_example.c

Lines 31 to 32 in e4285b5

	short buf[2*READ_SIZE];
	int ret = fread(buf, 2*sizeof(short), READ_SIZE, fin);

But according to fread documentation, although the 2nd and 3rd parameters will be multiplied together, the return value will be different(in case of full-read, the return value will be equal to 3rd parameter according to my test locally). So the variable ret will be different(prior is 256, current is 512, which equals to the length of buf).

Only this issue is fixed, the 3rd parameter of ope_encoder_write will write all data correctly. Otherwise it will write only half of the data, resulting in only half time length of the opus product in my local test(I'm using a 16000 sample rate, 1 channel pcm file for test. Already changed the ope_encoder_create_file parameters to 16000, 1, 0 for the local test).

Meanwhile, I'm not sure why we add a 2* with READ_SIZE for the buf, we can remove both 2* if it's useless.