Fix segfault in add_cc_sub_text and initialize to NULL in init_encoder (#950)

This commit adds some checks to avoid segmentation faults.

* In `add_cc_sub_text()`, strdup will cause a segfault if we duplicate an
  empty string.

* In `init_encoder()`, initialize pointer fields to NULL to avoid random
  addressing so we can avoid illegal memory accessing and segfaults in
  other places.

src/lib_ccx/ccx_common_common.c

@@ -70,6 +70,8 @@ void freep(void *arg)
 int add_cc_sub_text(struct cc_subtitle *sub, char *str, LLONG start_time,
 		LLONG end_time, char *info, char *mode, enum ccx_encoding_type e_type)
 {
+	if (str == NULL || strlen(str) == 0)
+		return 0;
 	if (sub->nb_data)
 	{
 		for(;sub->next;sub = sub->next);

src/lib_ccx/ccx_encoders_common.c

@@ -1038,6 +1038,7 @@ struct encoder_ctx *init_encoder(struct encoder_cfg *opt)
 	ctx->segment_last_key_frame = 0;
 	ctx->nospupngocr = opt->nospupngocr;
 
+	ctx->prev = NULL;
 	return ctx;
 }