mesa: use signed temporary variable to store _ColorDrawBufferIndexes

_ColorDrawBufferIndexes is defined as GLint* and using a GLuint*
will result in the first part of the conditional to be evaluated to
true always.

Unintentionally introduced by the following commit, this will result
in a driver segfault if one is using an old version of the piglit test

    bin/clearbuffer-mixed-format -auto -fbo

commit 03d848ea10
Author: Marek Olšák <marek.olsak@amd.com>
Date:   Wed Dec 4 00:27:20 2013 +0100

    mesa: fix interpretation of glClearBuffer(drawbuffer)

    This corresponding piglit tests supported this incorrect behavior instead of
    pointing at it.

Cc: Marek Olšák <marek.olsak@amd.com>
Cc: 10.0 9.2 9.1 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

src/mesa/main/clear.c

@@ -278,7 +278,7 @@ make_color_buffer_mask(struct gl_context *ctx, GLint drawbuffer)
       break;
    default:
       {
-         GLuint buf = ctx->DrawBuffer->_ColorDrawBufferIndexes[drawbuffer];
+         GLint buf = ctx->DrawBuffer->_ColorDrawBufferIndexes[drawbuffer];
 
          if (buf >= 0 && att[buf].Renderbuffer) {
             mask |= 1 << buf;