Issue error on erroneous expression

A while back I discovered that this does not issue an error:

    (gdb) p $x = (void * ) 57
    $3 = (void *) 0x39
    (gdb) p $x + 7 = 3
    $6 = (void *) 0x3

This patch fixes the bug.
Regression tested on x86-64 Fedora 36.

Reviewed-By: Bruno Larsen <blarsen@redhat.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=19312