AMDGPU: Fix crash when dumping unknown opcode

I'm for some reason having a problem producing a test.
It should be the same as test/MC/X86/invalid_opcode.s,
but llvm-mc seems to ignore random bytes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267225 91177308-0d34-0410-b5e6-96231b3b80d8

tools/llvm-objdump/llvm-objdump.cpp

@@ -405,6 +405,11 @@ public:
                  raw_ostream &OS,
                  StringRef Annot,
                  MCSubtargetInfo const &STI) override {
+    if (!MI) {
+      OS << " <unknown>";
+      return;
+    }
+
     SmallString<40> InstStr;
     raw_svector_ostream IS(InstStr);