Always reset file descriptor after consuming it

This prevents the request cancelling logic from closing an unrelated fd that happens to reuse the same number.

src/async.zsh

@@ -73,4 +73,5 @@ _zsh_autosuggest_async_response() {
 
 	# Always remove the handler
 	zle -F "$1"
+	_ZSH_AUTOSUGGEST_ASYNC_FD=
 }

zsh-autosuggestions.zsh

@@ -828,6 +828,7 @@ _zsh_autosuggest_async_response() {
 
 	# Always remove the handler
 	zle -F "$1"
+	_ZSH_AUTOSUGGEST_ASYNC_FD=
 }
 
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