Be sure to set variables to NULL after freeing them; doing otherwise can lead to...

Be sure to set variables to NULL after freeing them; doing otherwise can lead to referencing invalid memory later.  In this case, a particular series of callbacks (seen, for example, when signing onto Yahoo) could lead to a sourceInfo object remaining in the sourceInfoDict, keyed by a tag which has already been invalidated... and then an attempted double-free of its run loop source. Fixes #9026. Fixes #9025.

git-svn-id: svn://svn.adiumx.com/adium/trunk@22464 1c916fce-81e2-0310-a464-8ca513f45935