BUGFIX: Compilation error with -DNDEBUG defined.

When building a "MinSizeRel" with CMake I get a compilation error in lex_unget_unsave. This is because assertions are turned off using -DNDEBUG:

```
/usr/bin/gcc  -DHAVE_CONFIG_H -fPIC -Os -DNDEBUG -Ijansson/build/include -Ijansson/build/private_include    -Wall -Wextra -Wdeclaration-after-statement -Werror -o CMakeFiles/jansson.dir/src/load.c.o   -c jansson/src/load.c
jansson/src/load.c: In function âx_unget_unsaveâjansson/src/load.c:256:14: error: variable â set but not used [-Werror=unused-but-set-variable]
cc1: all warnings being treated as errors
```

This will then remove the insert, which makes the "d" variable unused, which is treated as an error since we have -Wall set. We can't simply get rid of the variable either and put the strbuffer_pop call in the assert call, since it's a macro and would remove the call entirely. So I simply added a check for NDEBUG to fix it.