advapi32: Do not copy string if destination pointer is NULL

[Andrew Talbot]

Changelog:
    advapi32: Do not copy string if destination pointer is NULL.

diff --git a/dlls/advapi32/security.c b/dlls/advapi32/security.c
index 333ad86..ec8f64a 100644
--- a/dlls/advapi32/security.c
+++ b/dlls/advapi32/security.c
@@ -1849,7 +1849,8 @@ LookupPrivilegeNameW( LPCWSTR lpSystemName, PLUID lpLuid, LPWSTR lpName,
     }
     else
     {
-        strcpyW(lpName, WellKnownPrivNames[lpLuid->LowPart]);
+        if (lpName)
+            strcpyW(lpName, WellKnownPrivNames[lpLuid->LowPart]);
         *cchName = privNameLen;
         return TRUE;
     }