Fix parameter numbers and missing alpha check for imagecolorset()

The check for the alpha parameter existed in PHP 7.4 but was lost in PHP 8.0.

Fixes: 50765075db6b6d5a9597589601c59f

Fix parameter numbers and missing alpha check for imagecolorset()

The check for the alpha parameter existed in PHP 7.4 but was lost in PHP 8.0.

Fixes: 50765075db6b6d5a9597589601c59f743f6ee9c8

Closes GH-14477.

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