screenshot: avoid confusing template error message with no file

Previously, with mpv --force-window=yes --idle=yes --screenshot-template="%f",
mpv would display an error saying that the template was incorrect, which it
isn't, there's just no file to put in the format. In this case, just use the
string "NO_FILE".

Author: kevmitch

player/screenshot.c

@@ -165,15 +165,17 @@ static char *create_fname(struct MPContext *mpctx, char *template,
         }
         case 'f':
         case 'F': {
-            if (!mpctx->filename)
-                goto error_exit;
-            char *video_file = mp_basename(mpctx->filename);
-            if (video_file) {
-                char *name = video_file;
-                if (fmt == 'F')
-                    name = stripext(res, video_file);
-                append_filename(&res, name);
-            }
+            char *video_file = NULL;
+            if (mpctx->filename)
+                video_file = mp_basename(mpctx->filename);
+
+            if (!video_file)
+                video_file = "NO_FILE";
+
+            char *name = video_file;
+            if (fmt == 'F')
+                name = stripext(res, video_file);
+            append_filename(&res, name);
             break;
         }
         case 'x':