asm ("" : : "m" (&key));
The best answer is really GCC's __asm__("" : : "m" (&key)), or perhaps something like __asm__("" : : "r" (key) : "memory"), after the memset. It generates no extra code, just ensures that the memset won't be removed.
For other compilers (in practice only MSVC, since clang is gcc-compatible), you could pass the pointer to a dummy assembly function instead of using inline assembly; even link-time optimization can't know what happens within a function written in assembly. Or, for better performance, create in assembly a "safer_memset" which is a single instruction: a jump to the real memset function.