MMU/PatchEngine: Fix potential crash during stack probe

TryReadInstruction doesn't validate the address it resolves, that
can result in Memory::GetPointer failing and returning nullptr
which then leads to a nullptr dereference and a crash.

Created PowerPC::HostIsInstructionRAMAddress which works the same
way as PowerPC::HostIsRAMAddress for the IBAT.

Source/Core/Core/PowerPC/PowerPC.h

@@ -220,6 +220,8 @@ void HostWrite_U64(const u64 var, const u32 address);
 // Returns whether a read or write to the given address will resolve to a RAM
 // access given the current CPU state.
 bool HostIsRAMAddress(const u32 address);
+// Same as HostIsRAMAddress, but uses IBAT instead of DBAT.
+bool HostIsInstructionRAMAddress(u32 address);
 
 std::string HostGetString(u32 em_address, size_t size = 0);