segfaults with the loop transport

[tbzatek]

FYI, just spotted a bunch of segfaults coming from the fact the loop transport's address sysfs value is empty:

libnvme/src/nvme/tree.c

Lines 2232 to 2233 in 6e6154a

	FREE_CTRL_ATTR(c->address);
	c->address = strdup(addr);

Introduced in #1033

Program received signal SIGSEGV, Segmentation fault.
Downloading source file /usr/src/debug/glibc-2.41.9000-24.fc43.x86_64/string/../sysdeps/x86_64/multiarch/strlen-avx2.S
__strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:76                                                                                                                                                                                                                                                             
76		VPCMPEQ	(%rdi), %ymm0, %ymm1
(gdb) t a a bt

Thread 1 (Thread 0x7ffff7b85840 (LWP 159833) "nvme"):
#0  __strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:76
#1  0x00007ffff7dc12e3 in __GI___strdup (s=s@entry=0x0) at strdup.c:41
#2  0x00007ffff7f90e93 in nvme_ctrl_alloc (r=0x5555556da6e0, s=0x5555556dc130, path=0x5555556d84f0 "/sys/class/nvme/nvme3", name=0x5555556da233 "nvme3") at ../src/nvme/tree.c:2233
#3  nvme_scan_ctrl (r=r@entry=0x5555556da6e0, name=0x5555556da233 "nvme3") at ../src/nvme/tree.c:2296
#4  0x00007ffff7f9150a in nvme_scan_topology (r=r@entry=0x5555556da6e0, f=f@entry=0x0, f_args=f_args@entry=0x0) at ../src/nvme/tree.c:298
#5  0x0000555555572e38 in list (argc=<optimized out>, argv=<optimized out>, cmd=<optimized out>, plugin=<optimized out>) at ../nvme.c:3598
#6  0x00005555555b8b91 in handle_plugin (argc=2, argv=0x7fffffffe180, plugin=0x5555556c8f00 <builtin>) at ../plugin.c:188
#7  0x000055555555b3f1 in main (argc=3, argv=0x7fffffffe178) at ../nvme.c:11068