Merge branch 'js/curl-off-t-fixes' into maint-2.51

Merge branch 'js/curl-off-t-fixes' into maint-2.51

A few places where an size_t value was cast to curl_off_t without
checking has been updated to use the existing helper function.

* js/curl-off-t-fixes:
  http-push: avoid new compile error
  imap-send: be more careful when casting to `curl_off_t`
  http: offer to cast `size_t` to `curl_off_t` safely