flt.lttng-utils.debug-info: use glib memory and string functions

flt.lttng-utils.debug-info: use glib memory and string functions

- Change use of glibc's malloc() to glib's g_new0() to follow the rest of
  the project.

- Change use of glibc string functions to their glib equivalent.

- Remove uses of GNU extension of conditional expressions with omitted
  operands as it would only be useful if the evaluation of the condition
  had side effects which it doesn't. Let's prefer code simplicity.

[0]:  https://gcc.gnu.org/onlinedocs/gcc-4.2.4/gcc/Conditionals.html

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>