Fix some LGTM errors of "Multiplication result converted to larger type"

core/io/file_access_compressed.cpp

@@ -233,7 +233,7 @@ uint64_t FileAccessCompressed::get_position() const {
 	if (writing) {
 		return write_pos;
 	} else {
-		return read_block * block_size + read_pos;
+		return (uint64_t)read_block * block_size + read_pos;
 	}
 }
 

core/io/file_access_zip.cpp

@@ -99,7 +99,7 @@ static int godot_testerror(voidpf opaque, voidpf stream) {
 }
 
 static voidpf godot_alloc(voidpf opaque, uInt items, uInt size) {
-	return memalloc(items * size);
+	return memalloc((size_t)items * size);
 }
 
 static void godot_free(voidpf opaque, voidpf address) {

core/io/zip_io.cpp

@@ -100,7 +100,7 @@ int zipio_testerror(voidpf opaque, voidpf stream) {
 }
 
 voidpf zipio_alloc(voidpf opaque, uInt items, uInt size) {
-	voidpf ptr = memalloc(items * size);
+	voidpf ptr = memalloc((size_t)items * size);
 	memset(ptr, 0, items * size);
 	return ptr;
 }