contrib/elf2dmp: Use g_malloc(), g_new() and g_free()

QEMU coding style uses the glib memory allocation APIs, not
the raw libc malloc/free. Switch the allocation and free
calls in elf2dmp to use these functions (dropping the now-unneeded
checks for failure).

Signed-off-by: Suraj Shirvankar <surajshirvankar@gmail.com>
Message-id: 169753938460.23804.11418813007617535750-1@git.sr.ht
[PMM: also remove NULL checks from g_malloc() calls;
 beef up commit message]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>