glsl, st/shader_cache: check the whole sha1 for zero

glsl, st/shader_cache: check the whole sha1 for zero

The checks were only looking at the first byte, while the intention
seems to be to check if the whole sha1 is zero. This prevented all
shaders with first byte zero in their sha1 from being saved.

This shaves around a second from Deus Ex load time on a hot cache.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Edward O'Callaghan <funfunctor@folklore1984.net>