encoding.c: Support calling peekc past the end of the buffer. This makes calling peekc easier.

src/libespeak-ng/encoding.c

 uint32_t
 text_decoder_peekc(espeak_ng_TEXT_DECODER *decoder)
 {
+	if (decoder->current == decoder->end) return 0;
+
 	const uint8_t *current = decoder->current;
 	uint32_t c = decoder->get(decoder);
 	decoder->current = current;

tests/encoding.c

 	assert(text_decoder_getc(decoder) == 'd');
 	assert(text_decoder_eof(decoder) == 1);
 
+	// Calling peekc past the end of the buffer.
+	assert(text_decoder_peekc(decoder) == '\0');
+	assert(text_decoder_eof(decoder) == 1);
+
 	destroy_text_decoder(decoder);
 }