Clear $phpmailer->AltBody to avoid unintended inheritance from previous wp_mail() calls

Since the wp_mail() function doesn’t clear the $phpmailer->AltBody attribute and uses the same $phpmailer object repetitively, it is necessary to clear any data that may have been stored in the attribute from previous wp_mail() calls.

This vulnerability affects the following application versions:

• Contact Form 7 4.4
• Contact Form 7 4.4.1
• Contact Form 7 4.4.2
• Contact Form 7 4.5
• Contact Form 7 4.5.1
• Contact Form 7 4.6
• Contact Form 7 4.6.1
• Contact Form 7 4.7
• Contact Form 7 4.8