CPU/Recompiler: Prevent using fastmem when cache is isolated

No point even trying since it's just going to fault.

src/core/cpu_core.cpp

@@ -99,6 +99,8 @@ void Initialize()
   s_last_breakpoint_check_pc = INVALID_BREAKPOINT_PC;
   s_single_step = false;
 
+  UpdateFastmemBase();
+
   GTE::Initialize();
 
   if (g_settings.gpu_pgxp_enable)
@@ -131,6 +133,7 @@ void Reset()
   g_state.cop0_regs.cause.bits = 0;
 
   ClearICache();
+  UpdateFastmemBase();
 
   GTE::Reset();
 
@@ -195,6 +198,14 @@ bool DoState(StateWrapper& sw)
   return !sw.HasError();
 }
 
+void UpdateFastmemBase()
+{
+  if (g_state.cop0_regs.sr.Isc)
+    g_state.fastmem_base = nullptr;
+  else
+    g_state.fastmem_base = Bus::GetFastmemBase();
+}
+
 ALWAYS_INLINE_RELEASE void SetPC(u32 new_pc)
 {
   DebugAssert(Common::IsAlignedPow2(new_pc, 4));
@@ -1973,14 +1984,6 @@ bool InterpretInstructionPGXP()
   return g_state.exception_raised;
 }
 
-void UpdateFastmemMapping()
-{
-  if (g_state.cop0_regs.sr.Isc)
-    g_state.fastmem_base = nullptr;
-  else
-    g_state.fastmem_base = Bus::GetFastmemBase();
-}
-
 } // namespace Recompiler::Thunks
 
 } // namespace CPU