get_device_info(): don't leak free()'ed pointer on error

[ydroneaud]

Programs using libwacom on a system without any supported device are being subject to a segfault:

malloc_consolidate(): unaligned fastbin chunk detected
Abandon (core dumped)

Using valgrind on list-local-devices shows the issue:

==29751== Command: ./list-local-devices
==29751==
==29751== Invalid free() / delete / delete[] / realloc()
==29751==    at 0x4842E43: free (vg_replace_malloc.c:989)
==29751==    by 0x48D8D44: g_free (gmem.c:208)
==29751==    by 0x485BD8B: g_autoptr_cleanup_generic_gfree (glib-autocleanups.h:32)
==29751==    by 0x485DCC2: libwacom_new_from_path (libwacom.c:790)
==29751==    by 0x4014B0: main (list-local-devices.c:301)
==29751==  Address 0x5234aa0 is 0 bytes inside a block of size 28 free'd
==29751==    at 0x4842E43: free (vg_replace_malloc.c:989)
==29751==    by 0x48D8D44: g_free (gmem.c:208)
==29751==    by 0x485C9ED: get_device_info (libwacom.c:369)
==29751==    by 0x485DBE9: libwacom_new_from_path (libwacom.c:799)
==29751==    by 0x4014B0: main (list-local-devices.c:301)
==29751==  Block was alloc'd at
==29751==    at 0x483FB26: malloc (vg_replace_malloc.c:446)
==29751==    by 0x48DE9E9: g_malloc (gmem.c:100)
==29751==    by 0x48F8AB9: g_strdup (gstrfuncs.c:323)
==29751==    by 0x485C4F9: g_strdup_inline (gstrfuncs.h:321)
==29751==    by 0x485C4F9: get_device_prop (libwacom.c:228)
==29751==    by 0x485C8C8: get_device_info (libwacom.c:342)
==29751==    by 0x485DBE9: libwacom_new_from_path (libwacom.c:799)
==29751==    by 0x4014B0: main (list-local-devices.c:301)
==29751==
Failed to find any devices known to libwacom.

In case of error in get_device_info(), pointer being released must not be returned to the caller.

[whot]

thanks!