Check for NULL before calling strlen.

Coverity 1397270
diff --git a/lib/zip_file_set_encryption.c b/lib/zip_file_set_encryption.c
index 91f9a27..06a25a9 100644
--- a/lib/zip_file_set_encryption.c
+++ b/lib/zip_file_set_encryption.c
@@ -88,7 +88,9 @@
 	
         if (e->changes == NULL) {
             if ((e->changes=_zip_dirent_clone(e->orig)) == NULL) {
-		_zip_crypto_clear(our_password, strlen(our_password));
+		if (our_password) {
+		    _zip_crypto_clear(our_password, strlen(our_password));
+		}
 		free(our_password);
                 zip_error_set(&za->error, ZIP_ER_MEMORY, 0);
                 return -1;