msan: notate more variable assignments from assembly code

This was missed in 31ba404944 because older
versions of clang did not complain about it. But clang-17, at least, does.

src/scalar_4x64_impl.h

@@ -814,6 +814,9 @@ static void secp256k1_scalar_mul_512(uint64_t *l8, const secp256k1_scalar *a, co
     : "+d"(pb)
     : "S"(l8), "D"(a->d)
     : "rax", "rbx", "rcx", "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15", "cc", "memory");
+
+    SECP256K1_CHECKMEM_MSAN_DEFINE(l8, sizeof(*l8) * 8);
+
 #else
     /* 160 bit accumulator. */
     uint64_t c0 = 0, c1 = 0;