attribute: Add a missing 'else'
attribute: Add a missing 'else'

The handler for the normal attribute was missing an else so presumably
it would have crashed on GLES2.

Reviewed-by: Robert Bragg <12e9293ec6b30c7fa8a0926af42807e929c1684f@linux.intel.com>

collects/cogl,djdeath/cogl,Distrotech/cogl,collects/cogl,gcampax/cogl,Distrotech/cogl,djdeath/cogl,djdeath/cogl-android,gcampax/cogl,Distrotech/cogl,djdeath/cogl-android,gcampax/cogl,Distrotech/cogl,djdeath/cogl-android,djdeath/cogl,gcampax/cogl