opt: Fix build issue with gcc 16

Compiling with gcc 16 throws this error:

    FAILED: [code=1] source/opt/CMakeFiles/SPIRV-Tools-opt.dir/decoration_manager.cpp.o
    source/opt/decoration_manager.cpp: In member function
      ‘spvtools::opt::analysis::DecorationManager::CloneDecorations(unsigned int, unsigned int)’:
    source/opt/decoration_manager.cpp:546:27: error:
      ‘MEM[(unsigned int &)&op + 24]’ may be used uninitialized [-Werror=maybe-uninitialized]

      546 |           if (op.words[0] == from) {  // add new pair of operands: (to, literal)
    source/opt/decoration_manager.cpp:545:19: note: ‘op’ declared here
      545 |           Operand op = inst->GetOperand(i);
          |                   ^~
    cc1plus: all warnings being treated as errors

Make sure that the vector is not empty before using it.

Author: jexposit

source/opt/decoration_manager.cpp

@@ -543,7 +543,7 @@ void DecorationManager::CloneDecorations(uint32_t from, uint32_t to) {
         const uint32_t num_operands = inst->NumOperands();
         for (uint32_t i = 1; i < num_operands; i += 2) {
           Operand op = inst->GetOperand(i);
-          if (op.words[0] == from) {  // add new pair of operands: (to, literal)
+          if (!op.words.empty() && op.words[0] == from) {  // add new pair of operands: (to, literal)
             inst->AddOperand(
                 Operand(spv_operand_type_t::SPV_OPERAND_TYPE_ID, {to}));
             op = inst->GetOperand(i + 1);