Fix type of return value.

From PVS-Studio via Michael Beck.
diff --git a/lib/zip_extra_field.c b/lib/zip_extra_field.c
index 58864cb..0350478 100644
--- a/lib/zip_extra_field.c
+++ b/lib/zip_extra_field.c
@@ -213,7 +213,7 @@
 
     if ((buffer = _zip_buffer_new((zip_uint8_t *)data, len)) == NULL) {
         zip_error_set(error, ZIP_ER_MEMORY, 0);
-        return NULL;
+        return false;
     }
     
     ef_head = ef = NULL;