IMAP literal size truncation on 32-bit platforms corrupts downloads and desynchronizes the response parser

On 32-bit builds of libcurl, the IMAP FETCH handler truncates a server-announced literal size (`{size}`) from `curl_off_t` (64-bit) to `size_t` (32-bit) when it decides how many already-buffered bytes belong to the literal body. When a malicious or malfunctioning IMAP server announces a literal …

/* lib/imap.c ~1272-1280 */
size_t chunk = after_header;          /* bytes already buffered after the header line */
...
if(chunk > (size_t)size)              /* <-- size (curl_off_t) truncated to size_t */
  chunk = (size_t)size;