[gdb/testsuite] Log on return in Term::_log_cur

[gdb/testsuite] Log on return in Term::_log_cur

Proc Term::_log_cur logs the cursor update of code in its body argument:
...
    proc _ctl_0x08 {} {
        _log_cur "Backspace" {
            variable _cur_col

            if {$_cur_col > 0} {
                incr _cur_col -1
            }
        }
    }
...
giving us for instance:
...
+++ Backspace, cursor: (2, 0) -> (2, 0)
...

But if we rewrite the code to use a return:
...
	    if { $_cur_col == 0 } {
		return
	    }
	    incr _cur_col -1
...
and the return is triggered, the log message disappears.

Fix this by wrapping the "uplevel $body" in a catch:
...
-	uplevel $body
+	set code [catch {uplevel $body} result]
...

Tested on aarch64-linux.