Writing for(;;) in a shader was crashing because
a NULL condition was dereferenced. Added the
NULL pointer check to fix the crash.

Change-Id: I2be7a4594029c928ff83221f65503636bc28f4a9
Reviewed-on: https://swiftshader-review.googlesource.com/3553Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>