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.

gdb/dtrace-probe.c

@@ -413,7 +413,7 @@ dtrace_process_dof_probe (struct objfile *objfile,
       for (j = 0; j < ret->probe_argc; j++)
 	{
 	  struct dtrace_probe_arg arg;
-	  struct expression *expr;
+	  struct expression *expr = NULL;
 
 	  /* Set arg.expr to ensure all fields in expr are initialized and
 	     the compiler will not warn when arg is used.  */