lavfi/drawtext: ignore final LF of textfile.

A standard text file ends with a final LF.
Without this change, it is interpreted as an empty final line,
and visible with the box option.
The current behavior can be achieved by actually having
an empty line at the end of the file.

Fix trac ticket #7948.

libavfilter/vf_drawtext.c

@@ -568,6 +568,11 @@ static int load_font(AVFilterContext *ctx)
     return err;
 }
 
+static inline int is_newline(uint32_t c)
+{
+    return c == '\n' || c == '\r' || c == '\f' || c == '\v';
+}
+
 static int load_textfile(AVFilterContext *ctx)
 {
     DrawTextContext *s = ctx->priv;
@@ -583,6 +588,8 @@ static int load_textfile(AVFilterContext *ctx)
         return err;
     }
 
+    if (textbuf_size > 0 && is_newline(textbuf[textbuf_size - 1]))
+        textbuf_size--;
     if (textbuf_size > SIZE_MAX - 1 || !(tmp = av_realloc(s->text, textbuf_size + 1))) {
         av_file_unmap(textbuf, textbuf_size);
         return AVERROR(ENOMEM);
@@ -595,11 +602,6 @@ static int load_textfile(AVFilterContext *ctx)
     return 0;
 }
 
-static inline int is_newline(uint32_t c)
-{
-    return c == '\n' || c == '\r' || c == '\f' || c == '\v';
-}
-
 #if CONFIG_LIBFRIBIDI
 static int shape_text(AVFilterContext *ctx)
 {