Use I3SOCK environment variable for path in AnyEvent::I3

Commit 3ae5f31 introduced the I3SOCK environment variable. This
prevents us from having to call `i3 --get-socketpath'. In case the
variable doesn't exist, fall back to the old ways.

Signed-off-by: Wesley Schwengle <wesleys@opperschaap.net>

AnyEvent-I3/lib/AnyEvent/I3.pm

@@ -182,6 +182,14 @@ instance on the current DISPLAY which is almost always what you want.
 sub new {
     my ($class, $path) = @_;
 
+    # We have I3SOCK now
+    $path ||= $ENV{I3SOCK};
+
+    # This is to untaint the path
+    if ($path && $path =~ /^([[:word:]\/\.]]+)$/) {
+        $path = $1;
+    }
+
     $path = _call_i3('--get-socketpath') unless $path;
 
     # This is the old default path (v3.*). This fallback line can be removed in
@@ -197,6 +205,10 @@ sub new {
         $path =~ s/~/$home/g;
     }
 
+    if(!-S $path) {
+        die "$path is not a socket", $/;
+    }
+
     bless { path => $path } => $class;
 }