Out of interest: how did you stumbl= e on that code?

Using the musl libc, getpass is not defined unless you enable _GNU_SOURC= E:

[ 75%] Building C object kcheckpass/CMakeFiles/kcheckpass.dir/kch=
eckpass.c.o
cd /usr/src/kde-plasma/kscreenlocker-5.8.5/work/kscreenlocker-5.8.5_build/k=
checkpass && /usr/bin/gcc  -D_LARGEFILE64_SOURCE -D_XOPEN_SOURCE=3D=
600 -I/usr/src/kde-plasma/kscreenlocker-5.8.5/work/kscreenlocker-5.8.5_buil=
d/kcheckpass -I/usr/src/kde-plasma/kscreenlocker-5.8.5/work/kscreenlocker-5=
.8.5/kcheckpass -I/usr/src/kde-plasma/kscreenlocker-5.8.5/work/kscreenlocke=
r-5.8.5_build   -DQT_NO_DEBUG -DNDEBUG -O2 -mlong-double-64 -ggdb -mcpu=3DG=
3 -fno-omit-frame-pointer  -std=3Diso9899:1990 -fno-common -Wall -Wextra -W=
cast-align -Wchar-subscripts -Wformat-security -Wno-long-long -Wpointer-ari=
th -Wundef -Wmissing-format-attribute -Wwrite-strings -Werror=3Dimplicit-fu=
nction-declaration -std=3Dgnu90 -fvisibility=3Dhidden    -U_REENTRANT -o CM=
akeFiles/kcheckpass.dir/kcheckpass.c.o -c /usr/src/kde-plasma/kscreenlocker=
-5.8.5/work/kscreenlocker-5.8.5/kcheckpass/kcheckpass.c
/usr/src/kde-plasma/kscreenlocker-5.8.5/work/kscreenlocker-5.8.5/kcheckpass=
/kcheckpass.c: In function =E2=80=98conv_legacy=E2=80=99:
/usr/src/kde-plasma/kscreenlocker-5.8.5/work/kscreenlocker-5.8.5/kcheckpass=
/kcheckpass.c:105:10: error: implicit declaration of function =E2=80=98getp=
ass=E2=80=99 [-Werror=3Dimplicit-function-declaration]
      p =3D getpass(prompt ? prompt : "Password: ");
          ^
/usr/src/kde-plasma/kscreenlocker-5.8.5/work/kscreenlocker-5.8.5/kcheckpass=
/kcheckpass.c:105:8: warning: assignment makes pointer from integer without=
 a cast [-Wint-conversion]
      p =3D getpass(prompt ? prompt : "Password: ");
        ^
cc1: some warnings being treated as errors
kcheckpass/CMakeFiles/kcheckpass.dir/build.make:62: recipe for target '=
;kcheckpass/CMakeFiles/kcheckpass.dir/kcheckpass.c.o' failed
make[2]: *** [kcheckpass/CMakeFiles/kcheckpass.dir/kcheckpass.c.o] Error 1
make[2]: Leaving directory '/usr/src/kde-plasma/kscreenlocker-5.8.5/wo=
rk/kscreenlocker-5.8.5_build'
CMakeFiles/Makefile2:2042: recipe for target 'kcheckpass/CMakeFiles/kc=
heckpass.dir/all' failed
make[1]: *** [kcheckpass/CMakeFiles/kcheckpass.dir/all] Error 2

If you think there is a risk: better be pedantic in this case. O= n the other hand getdelim should return -1 in error case and then your meth= od returns null. So in my book that's good enough error checking.
<= /blockquote>
My concern was on the off-chance that getdelim reads a partial password = but receives EOF before \n so it returns -1. But, looking at the standard,= there is no safe way to read/write to the buffer if the return code is -1,= so that is actually a moot point.