[trace] Avoid a crash in the dumper when disassembling fails

In rare situations, disassemblying would fail that produce an invalid
InstructionSP object. We need to check that it's valid before using.

With this change, now the dumper doesn't crash with dumping instructions of
ioctl. In fact, it now dumps this output

 {
    "id": 6135,
    "loadAddress": "0x7f4bfe5c7515",
    "module": "libc.so.6",
    "symbol": "ioctl",
    "source": "glibc/2.34/src/glibc-2.34/sysdeps/unix/syscall-template.S",
    "line": 120,
    "column": 0
 }

Anyway, we need to investigate why the diassembler failed disassembling that
instruction. From over 2B instructions I was disassembling today, just this
one failed, so this could be a bug in LLVM's core disassembler.

Differential Revision: https://reviews.llvm.org/D129588