Initialize EXPR in dtrace-probe::dtrace_process_dof_probe

Initialize EXPR in dtrace-probe::dtrace_process_dof_probe

GCC 4.4.7 generates the following warning:

 | cc1: warnings being treated as errors
 | dtrace-probe.c: In function ‘dtrace_process_dof_probe’:
 | dtrace-probe.c:416: error: ‘expr’ may be used uninitialized in this function
 | make[2]: *** [dtrace-probe.o] Error 1

Later versions (GCC 5) do a better job and don't generate the warning,
but it does not hurt to pre-initialize "expr" to NULL.

gdb/ChangeLog:

        * dtrace-probe.c (dtrace_process_dof_probe): Initialize expr to NULL.