nvmutil tmpdir: check world-writeable / sticky bits

must be world writeable and not have sticky bits

a bit theoretical, but we're also reading TMPDIR,
which could be anything

due to how this is called, it defaults back to /tmp
if null is returned, so itt's safe

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/nvmutil.h

@@ -28,6 +28,14 @@ int fchmod(int fd, mode_t mode);
 #define OFF_RESET 1
 #endif
 
+#ifndef S_ISVTX
+#define S_ISVTX 01000
+#endif
+
+#if defined(S_IFMT) && ((S_ISVTX & S_IFMT) != 0)
+#error "Unexpected bit layout"
+#endif
+
 #ifndef MAX_ZERO_RW_RETRY
 #define MAX_ZERO_RW_RETRY 5
 #endif