core: Zero-initialize wcore_info struct

Without this, it's possible that the garbage memory returned by malloc
will have wcore_info::is_init == true, causing wcore_tinfo_init() to
skip initialization.  That in turn causes a likely segfault the first
time wcore_info::error is accessed.

Fixes piglit crashes introduced by commit 6fcbe52 ("cmake: Refactor
compiler flags into new module").

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>