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

Bug: http://b/111067277
Test: builds
Change-Id: I8b69ea3815e14bb6eb27f40c0dd01a85b340a355
diff --git a/libdexfile/dex/dex_instruction.cc b/libdexfile/dex/dex_instruction.cc
index 8378211..83663c5 100644
--- a/libdexfile/dex/dex_instruction.cc
+++ b/libdexfile/dex/dex_instruction.cc
@@ -85,7 +85,7 @@
     default: LOG(FATAL) << "Tried to access the branch offset of an instruction " << Name() <<
         " which does not have a target operand.";
   }
-  return 0;
+  UNREACHABLE();
 }
 
 bool Instruction::CanFlowThrough() const {