PPCDebugInterface: Silence trivial -Wmaybe-uninitialized warning

We can just initialize register_index, even if it gets overwritten later
on via Common::FromChars.

Source/Core/Core/Debugger/PPCDebugInterface.cpp

@@ -467,7 +467,7 @@ PPCDebugInterface::GetMemoryAddressFromInstruction(const std::string& instructio
 
   if (is_reg == offset_match[0])
   {
-    unsigned register_index;
+    unsigned register_index = 0;
     Common::FromChars(offset_match.substr(1), register_index, 10);
     offset = (register_index == 0 ? 0 : m_system.GetPPCState().gpr[register_index]);
   }