Don't assign to a variable inside if statement in get_post_format()

[Mirucon]

Currently in the get_post_format() function, there's variable assign inside if statement. It is not a really good practice since assigning to a variable inside could confuse people as it looks like a mistake of the comparison operator. This patch will assign to a new variable just before the line of if().

[SergeyBiryukov]

In 43592:

Post Formats: Don't assign a variable inside if statement in get_post_format().

Props Mirucon.
Fixes #44840.