[gdb/tui] Fix Wmaybe-uninitialized in tui_find_disassembly_address

[gdb/tui] Fix Wmaybe-uninitialized in tui_find_disassembly_address

When building gdb with -O2, we run into:
...
gdb/tui/tui-disasm.c: In function ‘CORE_ADDR tui_find_disassembly_address \
  (gdbarch*, CORE_ADDR, int)’:
gdb/tui/tui-disasm.c:293:7: warning: ‘last_addr’ may be used uninitialized \
  in this function [-Wmaybe-uninitialized]
       if (last_addr < pc)
       ^~
...

The warning triggers since commit 72535eb14bd ("[gdb/tui] Fix segfault in
tui_find_disassembly_address").

Fix the warning by ensuring that last_addr is initialized at the point of
use:
...
+      last_addr = asm_lines.back ().addr;
       if (last_addr < pc)
...

Tested on x86_64-linux.