[gdb/testsuite] Fix cur_addr update in gdb.base/watchpoint-reuse-slot.exp

[gdb/testsuite] Fix cur_addr update in gdb.base/watchpoint-reuse-slot.exp

I noticed this code in gdb.base/watchpoint-reuse-slot.exp, proc stepi:
...
    gdb_test_multiple "stepi" $test {
	-re "($hex).*[string_to_regexp $srcline]\r\n$gdb_prompt $" {
	    set addr $expect_out(1,string)
	    if {$addr != $cur_addr} {
		pass $test
	    } else {
		fail $test
	    }
	    set cur_addr addr
	}
    }
...

The variable cur_addr is documented as:
...
 # The address the program is stopped at currently.
 set cur_addr ""
...
but in the gdb_test_multiple clause we assign the string "addr" to cur_addr,
while $addr contains the current address.

Fix this by assigning $addr instead "addr".

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-05-06  Tom de Vries  <tdevries@suse.de>

	* gdb.base/watchpoint-reuse-slot.exp: Fix incorrect assignment.