pidgin/pidgin

Fix an issue where color_is_visible was incorrectly comparing unsigned values that were potentially wrapping.

Testing Done:
Compile only.

Reviewed at https://reviews.imfreedom.org/r/493/
VOID:POINTER,POINTER,OBJECT
BOOLEAN:OBJECT,POINTER,STRING
VOID:STRING,STRING
VOID:STRING,STRING,DOUBLE
VOID:ENUM,STRING,STRING
VOID:ENUM,STRING,STRING,BOOLEAN
VOID:FLAGS,FLAGS
VOID:STRING,STRING,OBJECT,OBJECT
VOID:POINTER,POINTER,OBJECT,OBJECT
VOID:STRING,STRING,POINTER