glsl/blob: add valgrind checks that written data is defined

glsl/blob: add valgrind checks that written data is defined

Undefined data will eventually trigger a valgrind error while computing
its CRC32 while writing it into the disk cache, but at that point, it is
basically impossible to track down where the undefined data came from.

With this change, finding the origin of undefined data becomes easy.

v2: remove duplicate VALGRIND_CFLAGS (Emil)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>