Coding Standards: Fix instances of WordPress.PHP.NoSilencedErrors.Discouraged.

Noteable changes:
- The `magic_quotes_runtime` and `magic_quotes_sybase` settings were removed in PHP 5.4, so no longer need to be set.
- Some functions that use external libraries can generate errors that can't be tested for, so are globally allowed to silence errors.
- Quite a few functions would cause errors if `safe_mode` was set. This setting was removed in PHP 5.4.
- Only a handful of `header()` calls needed corresponding `headers_sent()` checks for unit tests to pass, but more may need to be added as the nightlies builds are tested.

See #46732.

Built from https://develop.svn.wordpress.org/trunk@45611


git-svn-id: http://core.svn.wordpress.org/trunk@45422 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/media.php

@@ -238,7 +238,7 @@ function image_downsize( $id, $size = 'medium' ) {
 		$info       = null;
 
 		if ( $thumb_file ) {
-			$info = getimagesize( $thumb_file );
+			$info = @getimagesize( $thumb_file );
 		}
 
 		if ( $thumb_file && $info ) {
@@ -910,8 +910,8 @@ function wp_get_attachment_image_src( $attachment_id, $size = 'thumbnail', $icon
 				/** This filter is documented in wp-includes/post.php */
 				$icon_dir = apply_filters( 'icon_dir', ABSPATH . WPINC . '/images/media' );
 
-				$src_file                = $icon_dir . '/' . wp_basename( $src );
-				@list( $width, $height ) = getimagesize( $src_file );
+				$src_file               = $icon_dir . '/' . wp_basename( $src );
+				list( $width, $height ) = @getimagesize( $src_file );
 			}
 		}