examples/fopen: fix comparison

As want is size_t, (file->buffer_pos - want) is unsigned, so checking
if it's less than zero makes no sense.
Check if file->buffer_pos is less than want instead to avoid the
unsigned integer wraparound.

Closes https://github.com/curl/curl/pull/3975

1 files changed, 1 insertions, 1 deletions

diff --git a/docs/examples/fopen.c b/docs/examples/fopen.c
index f1706fbe6..7151addde 100644
--- a/docs/examples/fopen.c
+++ b/docs/examples/fopen.c
@@ -211,7 +211,7 @@ static int fill_buffer(URL_FILE *file, size_t want)
 static int use_buffer(URL_FILE *file, size_t want)
 {
   /* sort out buffer */
-  if((file->buffer_pos - want) <= 0) {
+  if(file->buffer_pos <= want) {
     /* ditch buffer - write will recreate */
     free(file->buffer);
     file->buffer = NULL;