Fix sanitizer in egl_display_t::get.

On integer overflow sanitized builds, uintptr_t(dpy)-1U is causing a
runtime error:

 runtime error: unsigned integer overflow: 0 - 1 cannot be represented in
 type 'unsigned int'

This handles the case where dpy is 0 before the decrement occurs.

Bug: 30969751
Test: Compiles.
Change-Id: Ieafe818addfa09cf5b50e2df8f078296c5e3c759