Fixed crash caused by failure to allocate memory block of very specia…

…l size

Allocation of 810 FState objects (32400 bytes in total) could be performed in MSVC x64 build but worked in 32-bit version
This fixes https://mantis.zdoom.org/view.php?id=602

src/memarena.cpp

@@ -255,7 +255,7 @@ FMemArena::Block *FMemArena::AddBlock(size_t size)
 
 void FMemArena::Block::Reset()
 {
-	Avail = RoundPointer(this + sizeof(*this));
+	Avail = RoundPointer(reinterpret_cast<char*>(this) + sizeof(*this));
 }
 
 //==========================================================================