argpar.{c,h}: fix clang-tidy issues

argpar.{c,h}: fix clang-tidy issues

Fix these issues:

    /home/smarchi/src/argpar/argpar/argpar.h:344:9: warning: macro argument should be enclosed in parentheses [bugprone-macro-parentheses]
      344 |         _item = NULL;                                                                              \
          |         ^
          |         (    )
    argpar.c:525:34: warning: 'iter->tmp_buf.data' may be set to null if 'realloc' fails, which may result in a leak of the original buffer [bugprone-suspicious-realloc-usage]
      525 |             iter->tmp_buf.data = ARGPAR_REALLOC(iter->tmp_buf.data, char, iter->tmp_buf.size);
          |             ~~~~~~~~~~~~~~~~~~   ^
    argpar.c:15:56: note: expanded from macro 'ARGPAR_REALLOC'
       15 | #define ARGPAR_REALLOC(_ptr, _type, _nmemb) ((_type *) realloc(_ptr, (_nmemb) * sizeof(_type)))
          |                                                        ^       ~~~~

Change-Id: I2e42059f8c4c744e10bdbc7c78b2628a53cd60d7
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>