avfilter/vf_drawtext: use replacement chars for invalid UTF8 sequences

continue is explicitly disallowed for GET_UTF8, so let's fix that as well.
Fixes crash with invalid UTF8 sequences.
Signed-off-by: Marton Balint <cus@passwd.hu>

libavfilter/vf_drawtext.c

@@ -1225,7 +1225,8 @@ static int draw_glyphs(DrawTextContext *s, AVFrame *frame,
     for (i = 0, p = text; *p; i++) {
         FT_Bitmap bitmap;
         Glyph dummy = { 0 };
-        GET_UTF8(code, *p++, continue;);
+        GET_UTF8(code, *p++, code = 0xfffd; goto continue_on_invalid;);
+continue_on_invalid:
 
         /* skip new line chars, just go to new line */
         if (code == '\n' || code == '\r' || code == '\t')
@@ -1363,7 +1364,8 @@ static int draw_text(AVFilterContext *ctx, AVFrame *frame,
 
     /* load and cache glyphs */
     for (i = 0, p = text; *p; i++) {
-        GET_UTF8(code, *p++, continue;);
+        GET_UTF8(code, *p++, code = 0xfffd; goto continue_on_invalid;);
+continue_on_invalid:
 
         /* get glyph */
         dummy.code = code;
@@ -1386,7 +1388,8 @@ static int draw_text(AVFilterContext *ctx, AVFrame *frame,
     /* compute and save position for each glyph */
     glyph = NULL;
     for (i = 0, p = text; *p; i++) {
-        GET_UTF8(code, *p++, continue;);
+        GET_UTF8(code, *p++, code = 0xfffd; goto continue_on_invalid2;);
+continue_on_invalid2:
 
         /* skip the \n in the sequence \r\n */
         if (prev_code == '\r' && code == '\n')