Initialize member Instruction::index_.

Reported as unitialized when used here:
http://google3/wireless/android/security/exploits/stasys/dataflow_evaluator.cc?l=799&rcl=419853748

Found using:
-Xclang -enable-noundef-analysis
-mllvm -msan-eager-checks=1

PiperOrigin-RevId: 420954999
Change-Id: I9b182ef3201ef6ef1b13d8925a64c0da6352fc89

reader/instruction.cc

@@ -17,7 +17,7 @@
 namespace security::binexport {
 
 Instruction::Instruction(Address address, const std::string& mnemonic)
-    : address_(address), mnemonic_(mnemonic) {}
+    : address_(address), mnemonic_(mnemonic), index_(0) {}
 
 void Instruction::set_operands(const std::vector<int>& operand_indices) {
   operand_indices_ = operand_indices;