fix read of 8192 bytes failed with errno=21 Is a directory

[weierophinney]

With 7.4.0RC3

There was 1 error:
1) ZendTest\Mail\Storage\MboxFolderTest::testChangeFolderUnselectable
fgets(): read of 8192 bytes failed with errno=21 Is a directory
/builddir/build/BUILDROOT/php-zendframework-zend-mail-2.10.0-5.fc32.noarch/usr/share/php/Zend/Mail/Storage/Mbox.php:232
/builddir/build/BUILDROOT/php-zendframework-zend-mail-2.10.0-5.fc32.noarch/usr/share/php/Zend/Mail/Storage/Mbox.php:268
/builddir/build/BUILDROOT/php-zendframework-zend-mail-2.10.0-5.fc32.noarch/usr/share/php/Zend/Mail/Storage/Folder/Mbox.php:164
/builddir/build/BUILD/zend-mail-d7beb63d5f7144a21ac100072c453e63860cdab8/test/Storage/MboxFolderTest.php:143

This is a trivial fix which "also" mute E_NOTICE raised by fgets

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Originally posted by @remicollet at zendframework/zend-mail#244