mesa: use elementBuf local var instead of ctx->Array.ElementArrayBufferObj

Makes no real difference, but more consistant.
diff --git a/src/mesa/main/api_validate.c b/src/mesa/main/api_validate.c
index 42d1e57..1986cbe 100644
--- a/src/mesa/main/api_validate.c
+++ b/src/mesa/main/api_validate.c
@@ -72,7 +72,7 @@
    if (map) {
       ctx->Driver.UnmapBuffer(ctx,
                               GL_ELEMENT_ARRAY_BUFFER_ARB,
-                              ctx->Array.ElementArrayBufferObj);
+                              elementBuf);
    }
 
    return max;