C++17 compatibility: make WITH_TIDY=1 happy again.

Bug: http://b/111067277
Test: builds
Change-Id: I8b69ea3815e14bb6eb27f40c0dd01a85b340a355
diff --git a/disassembler/disassembler.cc b/disassembler/disassembler.cc
index aee690e..0662334 100644
--- a/disassembler/disassembler.cc
+++ b/disassembler/disassembler.cc
@@ -75,7 +75,7 @@
 #endif
     default:
       UNIMPLEMENTED(FATAL) << static_cast<uint32_t>(instruction_set);
-      return nullptr;
+      UNREACHABLE();
   }
 }