stb_image_write: prevent int to overflow before converting it to size_t

[illwieckz]

This fixes some warnings reported by CodeQL:

Multiplication result converted to larger type
Multiplication result may overflow 'int' before it is converted to 'size_t'. 

You may find details about the CodeQL reports in the comments made by the github-advanced-security bot in this thread:

• https://github.com/DaemonEngine/crunch/pull/53