Don't use string literals in assert()

Fixes #196.
12 jobs for master in 9 minutes and 26 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed build-debian #238693
debian amd64

00:00:32

passed build-debian-aarch64 #238697
aarch64 debian

00:01:28

passed build-debian-aarch64-clang-5 #238698
aarch64 debian

00:01:06

passed build-debian-static #238694
debian amd64

00:00:31

passed build-debian-werror #238700
aarch64 debian

00:00:31

passed build-macos #238699
macos

00:00:25

passed build-win32 #238695
win32

00:00:30

passed build-win64 #238696
win64

00:00:35

 
  Test
passed test-debian #238701
debian amd64

00:00:49

passed test-debian-asan #238702
debian amd64

00:02:16

passed test-debian-msan #238703
debian amd64

00:01:05

passed test-debian-ubsan #238704
debian amd64

00:01:25