shared/log: Fix not checking vasprintf return

shared/log: Fix not checking vasprintf return

It seems like some implementation of vasprintf set the content of the
str to NULL rather then returning -1 causing the following errors:

=================================================================
==216204==ERROR: AddressSanitizer: attempting free on address which
was not malloc()-ed: 0x55e787722cf0 in thread T0
      #0 0x55e784f75872 in __interceptor_free.part.0 asan_malloc_linux.cpp.o
      #1 0x55e7850e55f9 in bt_log_vprintf
/usr/src/debug/bluez-git/bluez-git/src/shared/log.c:154:2
      #2 0x55e78502db18 in monitor_log
/usr/src/debug/bluez-git/bluez-git/src/log.c:40:2
      #3 0x55e78502dab4 in info
/usr/src/debug/bluez-git/bluez-git/src/log.c:52:2
      #4 0x55e78502e314 in __btd_log_init
/usr/src/debug/bluez-git/bluez-git/src/log.c:179:2
      #5 0x55e78502aa63 in main
/usr/src/debug/bluez-git/bluez-git/src/main.c:1388:2
      #6 0x7f1d5fe27ccf  (/usr/lib/libc.so.6+0x27ccf) (BuildId:
316d0d3666387f0e8fb98773f51aa1801027c5ab)
      #7 0x7f1d5fe27d89 in __libc_start_main
(/usr/lib/libc.so.6+0x27d89) (BuildId:
316d0d3666387f0e8fb98773f51aa1801027c5ab)
      #8 0x55e784e88084 in _start
(/usr/lib/bluetooth/bluetoothd+0x36084) (BuildId:
19348ea642303b701c033d773055becb623fe79a)
  Address 0x55e787722cf0 is a wild pointer inside of access range of
size 0x000000000001.
  SUMMARY: AddressSanitizer: bad-free asan_malloc_linux.cpp.o in
__interceptor_free.part.0
  ==216204==ABORTING
сен 18 13:10:02 archlinux systemd[1]: bluetooth.service: Main process
exited, code=exited, status=1/FAILURE