libelektra: augeas plugin might crash

When doing kdb ls I got a crash in Augeas:

==14172== Callgrind, a call-graph generating cache profiler
==14172== Copyright (C) 2002-2017, and GNU GPL'd, by Josef Weidendorfer et al.
==14172== Using Valgrind-3.14.0 and LibVEX; rerun with -h for copyright info
==14172== Command: kdb ls /
==14172== 
==14172== For interactive control, run 'callgrind_control -h'.
==14172== brk segment overflow in thread #1: can't grow to 0x4830000
==14172== (see section Limitations in user manual)
==14172== NOTE: further instances of this message will not be shown
free(): double free detected in tcache 2

Sorry, I crashed by the signal SIGABRT
This should not have happened!

Please report the issue at https://issues.libelektra.org/
==14172== 
==14172== Process terminating with default action of signal 6 (SIGABRT): dumping core
==14172==    at 0x4B097BB: raise (raise.c:51)
==14172==    by 0x4AF4534: abort (abort.c:79)
==14172==    by 0x1D631B: catchSignal(int) (main.cpp:110)
==14172==    by 0x4B0983F: ??? (in /lib/x86_64-linux-gnu/libc-2.28.so)
==14172==    by 0x4B097BA: raise (internal-signals.h:84)
==14172==    by 0x4AF4534: abort (abort.c:79)
==14172==    by 0x4B4B507: __libc_message (libc_fatal.c:181)
==14172==    by 0x4B51C19: malloc_printerr (malloc.c:5341)
==14172==    by 0x4B536FC: _int_free (malloc.c:4193)
==14172==    by 0x4B41880: fclose@@GLIBC_2.2.5 (iofclose.c:77)
==14172==    by 0x4F09313: elektraAugeasGet (augeas.c:540)
==14172==    by 0x4908B3C: kdbGet (kdb.c:554)
==14172== 
==14172== Events    : Ir
==14172== Collected : 30693614075
==14172== 
==14172== I   refs:      30,693,614,075
zsh: abort      valgrind --tool=callgrind kdb ls /

The problem was quite obviously introduced by 339d34dfa331926e486340c6343b62266fb28e14 which does not return correctly in the error cases anymore.

A code review is needed if other places with return in macros are affected.