d3dx9_36/tests: Fix printing a NULL string

Bruno Haible was so kind to provide the following test run results:

glibc          SIGSEGV
MacOS X 10.5   (null)
FreeBSD 6.4    (null)
OpenBSD 4.9    (null)
AIX 7.1        empty
HP-UX 11.31    empty
IRIX 6.5       (null)
OSF/1 5.1      (null)
Solaris 10     SIGSEGV
Cygwin         (null)
mingw          (null)

for printf("%s", NULL)

Looks like the next Coccinelle job will be to flag potential NULL pointers
used in %s in logs, not the tests...