util: annotate that our mask cannot be NULL

Poor clang-tidy thinks that there's a code-path here that lets us
return with a NULL mask.

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1292>

src/util-bits.h

@@ -265,6 +265,9 @@ _nonnull_(1) static inline void _infmask_ensure_size(infmask_t *mask, unsigned i
 			mask->mask[i] = bitmask_new();
 		mask->nmasks = required;
 	}
+	/* for clang-tidy, poor thing gets confused otherwise */
+	assert(mask->mask);
+	assert(mask->nmasks >= 1);
 }
 
 static inline infmask_t