MDL-64075 antivirus_clamav: fix array_search usage

If 'unix' is the first element of the stream_get_transports() result
array, array_search() will return `0` as index result which, when
negated, will result as true.

lib/antivirus/clamav/adminlib.php

@@ -57,7 +57,7 @@ public function write_setting($data) {
     public function validate($data) {
         if ($data === 'unixsocket') {
             $supportedtransports = stream_get_transports();
-            if (!array_search('unix', $supportedtransports)) {
+            if (array_search('unix', $supportedtransports) === false) {
                 return get_string('errornounixsocketssupported', 'antivirus_clamav');
             }
         }