mime: fix the binary encoder to handle large data properly

New test 666 checks this is effective.
As upload buffer size is significant in this kind of tests, shorten it
in similar test 652.

Fixes #4860
Closes #4833
Reported-by: RuurdBeerstra on github

lib/mime.c

@@ -366,7 +366,7 @@ static size_t encoder_nop_read(char *buffer, size_t size, bool ateof,
     size = insize;
 
   if(size)
-    memcpy(buffer, st->buf, size);
+    memcpy(buffer, st->buf + st->bufbeg, size);
 
   st->bufbeg += size;
   return size;