Improve (?) handling of invalid addresses in Memset to avoid a crash

Core/MemMap.cpp

@@ -493,8 +493,13 @@ void Memset(const u32 _Address, const u8 _iValue, const u32 _iLength, const char
 		uint8_t *ptr = GetPointerWriteUnchecked(_Address);
 		memset(ptr, _iValue, _iLength);
 	} else {
-		for (size_t i = 0; i < _iLength; i++)
-			Write_U8(_iValue, (u32)(_Address + i));
+		// TODO: This mainly seems to be produced by GPUCommon::PerformMemorySet, called from
+		// Replace_memset_jak(). Strangely, this managed to crash in Write_U8().
+		for (size_t i = 0; i < _iLength; i++) {
+			if (Memory::IsValidAddress(_Address + i)) {
+				WriteUnchecked_U8(_iValue, (u32)(_Address + i));
+			}
+		}
 	}
 
 	if (tag) {