Fix thinko on dtrace-probe.c:dtrace_process_dof_probe

Fix thinko on dtrace-probe.c:dtrace_process_dof_probe

While investigating PR gdb/22557 ("Regression:
gdb.base/dtrace-probe.exp"), I noticed that the code is wrongly
declaring a new "expression_up" variable inside the TRY block in
"dtrace_process_dof_probe".  This causes the outter "expr" variable to
be empty, which may have an impact later when evaluating the
expression.

This commit fixes that.  Unfortunately the script used to test DTrace
probes (gdb/testsuite/lib/pdtrace.in) is not very reliable so I cannot
say whether this commit fixes the PR mentioned above.  Nonetheless,
it's an obvious fix and should go in.

gdb/ChangeLog:
2017-12-08  Sergio Durigan Junior  <sergiodj@redhat.com>

	* dtrace-probe.c (dtrace_process_dof_probe): Do not declare a new
	"expression_up" inside the TRY block.