Fix shocking memory leak on Windows

Spawning processes would leak lots of memory, due to a missing free call
in ntspawn(). Our tooling never caught this since ntspawn() must use the
WIN32 memory allocator. It means every time posix_spawn, fork, or execve
got called, we would leak 162kb of memory. I'm proud to say that's fixed

Author: jart

libc/calls/ntspawn.c

@@ -238,6 +238,7 @@ textwindows int ntspawn(struct NtSpawnArgs *args) {
   BLOCK_SIGNALS;
   if ((sb = ntspawn_malloc(sizeof(*sb)))) {
     rc = ntspawn2(args, sb);
+    ntspawn_free(sb);
   } else {
     rc = -1;
   }