Avoid segmentation faults in dinit_encoder() and add_cc_sub_text()

This commit adds some checks to avoid segmentation faults.

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

* In `dinit_encoder()`, accessing memory that doesn't exist will cause a
  segfault in case of processing hardsubs.

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

@@ -924,7 +924,8 @@ void dinit_encoder(struct encoder_ctx **arg, LLONG current_fts)
 		write_subtitle_file_footer(ctx, ctx->out + i);
 	}
 
-	free_encoder_context(ctx->prev);
+	if(!ccx_options.hardsubx) // Don't free ctx->prev in case of hardsubx to avoid segfault
+		free_encoder_context(ctx->prev);
 	dinit_output_ctx(ctx);
 	freep(&ctx->subline);
 	freep(&ctx->buffer);