Fix eof handling in a middle of task list item.

src/md4c.c

@@ -6141,7 +6141,7 @@ md_analyze_line(MD_CTX* ctx, OFF beg, OFF* p_end,
                 task_container->is_task = TRUE;
                 task_container->task_mark_off = tmp + 1;
                 off = tmp + 3;
-                while(ISWHITESPACE(off))
+                while(off < ctx->size  &&  ISWHITESPACE(off))
                     off++;
                 line->beg = off;
             }