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