In unsetenv() check for NULL or empty name as per POSIX 1003.1-2008

plugins/sudoers/env.c

@@ -330,7 +330,7 @@ unsetenv(const char *var)
     char **ep = env.envp;
     size_t len;
 
-    if (strchr(var, '=') != NULL) {
+    if (var == NULL || *var == '\0' || strchr(var, '=') != NULL) {
 	errno = EINVAL;
 #ifdef UNSETENV_VOID
 	return;