[cctools] fix ar(1) crash without argument, closes #7.

[cctools] fix ar(1) crash without argument, closes #7.

In ar(1), strcmp() checks are used to determine the value
of argument argv[1], even when no argument is given. In the
past, they were possibly harmless out-of-bound reads and
comparison with garbage, without consequences.

However, running it on macOS 13 w/ Apple Silicon immediately
crashes it with Segmentation Fault, because argv[1] is now
NULL and generates EXC_BAD_ACCESS in strcmp().

This commit checks whether argc is equal or greater than 2
before doing strcmp().

$ ./bin/ar
Segmentation fault: 11

* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
    frame #0: 0x0000000193181460 libsystem_platform.dylib`_platform_strcmp + 144
libsystem_platform.dylib`:
->  0x193181460 <+144>: ldr    q0, [x0], #0x10
    0x193181464 <+148>: ldr    q1, [x1], #0x10
    0x193181468 <+152>: cmeq.16b v1, v0, v1
    0x19318146c <+156>: and.16b v0, v0, v1
Target 0: (ar) stopped.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
  * frame #0: 0x0000000193181460 libsystem_platform.dylib`_platform_strcmp + 144
    frame #1: 0x0000000100006440 ar`main(argc=1, argv=0x000000016fdfef58) at ar.c:126:8
    frame #2: 0x0000000192e2be50 dyld`start + 2544

Signed-off-by: Yifeng Li <tomli@tomli.me>