glsl: Initialize per_vertex_accumulator::fields.

Fixes "Uninitialized pointer field" defect reported by Coverity.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>

src/glsl/builtin_variables.cpp

@@ -311,7 +311,8 @@ private:
 
 
 per_vertex_accumulator::per_vertex_accumulator()
-   : num_fields(0)
+   : fields(),
+     num_fields(0)
 {
 }