Style: Replaces uses of 0/NULL by nullptr (C++11)

Using clang-tidy's `modernize-use-nullptr`.
https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-nullptr.html

core/os/memory.h

@@ -137,13 +137,13 @@ void memdelete_allocator(T *p_class) {
 template <typename T>
 T *memnew_arr_template(size_t p_elements, const char *p_descr = "") {
 	if (p_elements == 0)
-		return 0;
+		return nullptr;
 	/** overloading operator new[] cannot be done , because it may not return the real allocated address (it may pad the 'element count' before the actual array). Because of that, it must be done by hand. This is the
 	same strategy used by std::vector, and the PoolVector class, so it should be safe.*/
 
 	size_t len = sizeof(T) * p_elements;
 	uint64_t *mem = (uint64_t *)Memory::alloc_static(len, true);
-	T *failptr = 0; //get rid of a warning
+	T *failptr = nullptr; //get rid of a warning
 	ERR_FAIL_COND_V(!mem, failptr);
 	*(mem - 1) = p_elements;