Fix moving position indicator out of bounds in FileAccessMemory

(cherry picked from commit bff0c71e2e2cb9b880181cc819c4753abfe59508)

core/io/file_access_memory.cpp

@@ -148,7 +148,7 @@ uint64_t FileAccessMemory::get_buffer(uint8_t *p_dst, uint64_t p_length) const {
 	}
 
 	memcpy(p_dst, &data[pos], read);
-	pos += p_length;
+	pos += read;
 
 	return read;
 }
@@ -176,7 +176,7 @@ void FileAccessMemory::store_buffer(const uint8_t *p_src, uint64_t p_length) {
 	}
 
 	memcpy(&data[pos], p_src, write);
-	pos += p_length;
+	pos += write;
 }
 
 FileAccessMemory::FileAccessMemory() {