[pvr.wmc] use safe format arg to snprintf

Prior to this commit, the wmc PVR addon failed to build if `-Werror=format-security` was used. The compiler could not examine the contents of the results array.

Adjust the `snprintf` calls so that the format string is a string literal.

Fedora recently enabled this compiler option in their RPM builds, so this fixes the build on Fedora 21.