Fix heap buffer overflow in UsageMonitor

[serebit]

Description

The %ms format specifier, when passed to POSIX scanf, allocates the necessary space for the value that it reads into a char**.

Submitter Checklist

• [x] Squashed commits with git rebase -i (if needed)
• [x] Built budgie-desktop and verified that the patch worked (if needed)

[EbonJaeger]

How portable is this? It looks like it's an extension done by GNU C, and I don't see any documentation, at least for OpenBSD that supports %m.

[serebit]

Hm... It's not portable. I'll need to figure out how to preallocate the required space, then.