[lldb] Correct unsigned decimal argument check in memory write

getAsInteger returns false when it succeeds.

Before:
(lldb) memory write 0x00007ffff7dd3000 99 -f "unsigned decimal"
error: '99' is not a valid unsigned decimal string value.

After:
(lldb) memory write 0x00007ffff7dd3000 99 -f "unsigned decimal"
(lldb) memory read 0x00007ffff7dd3000 0x00007ffff7dd3001
0x7ffff7dd3000: 63                                               c