efi: libstub: only free priv.runtime_map when allocated

In the Linux kernel, the following vulnerability has been resolved: efi: libstub: only free priv.runtime_map when allocated priv.runtime_map is only allocated when efi_novamap is not set. Otherwise, it is an uninitialized value. In the error path, it is freed unconditionally. Avoid passing an uninitialized value to free_pool. Free priv.runtime_map only when it was allocated. This bug was discovered and resolved using Coverity Static Analysis Security Testing (SAST) by Synopsys, Inc.