mesa: Returns a GL_INVALID_VALUE error if num of fbos in glDeleteRenderbuffers is negative

Per GLES3 manual for glDeleteRenderbuffers
<https://www.khronos.org/opengles/sdk/docs/man3/html/glDeleteRenderbuffers.xhtml>,
GL_INVALID_VALUE is generated if n is negative.

Fixes 1 dEQP test:
* dEQP-GLES3.functional.negative_api.buffer.delete_renderbuffers

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/main/fbobject.c

@@ -1311,6 +1311,11 @@ _mesa_DeleteRenderbuffers(GLsizei n, const GLuint *renderbuffers)
    GLint i;
    GET_CURRENT_CONTEXT(ctx);
 
+   if (n < 0) {
+      _mesa_error(ctx, GL_INVALID_VALUE, "glDeleteRenderbuffers(n < 0)");
+      return;
+   }
+
    FLUSH_VERTICES(ctx, _NEW_BUFFERS);
 
    for (i = 0; i < n; i++) {