llvmpipe: Check thread creation errors

llvmpipe: Check thread creation errors

In the case of glibc, pthread_t is internally a pointer.  If
lp_rast_destroy() passes a 0-value pthread_t to pthread_join(), the
latter will SEGV dereferencing it.

pthread_create() can fail if either the user's ulimit -u or Linux
kernel's /proc/sys/kernel/threads-max is reached.

Choosing to continue, rather than fail, on theory that it is better to
run with the one main thread, than not run at all.

Keeping as many threads as we got, since lack of threads severely
degrades llvmpipe performance.

Signed-off-by: Nathan Kidd <nkidd@opentext.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>