Don't abort the menu editor on sigwinch. (closes #3875)

Don't abort the menu editor on sigwinch. (closes #3875)

getch() will return ERR on sigwinch when timeout() is called with a
positive value.  mutt_getch() will therefore return ch==-2 for both a
timeout and a sigwinch in this case.

The imap code in km_dokey() exits out of the ImapKeepalive loop for a
SigWinch, and was skipping past the check for MENU_EDITOR and
tmp.ch==-2.  Move this check below the gotkey: label so the
ImapKeepalive loop behaves the same as the Timeout code.

Thanks to nicop for reporting the problem and for the initial patch!