segment faulty occured in add_feature()

Problem:
Multipathd segment faulty occured when device policy was configured in multipath.conf as follow:
#This is a device configuration just for COMPELNT device
devices {
       device {
               vendor                  COMPELNT
               dev_loss_tmo            120
               scheduler               deadline
       }
}

The call trace is:
Program received signal SIGSEGV, Segmentation fault.
0x00007f4eaaf49251 in __strstr_sse2 () from /lib64/libc.so.6
(gdb) bt
#0  0x00007f4eaaf49251 in __strstr_sse2 () from /lib64/libc.so.6
#1  0x00007f4eabba879c in add_feature (f=f@entry=0x211b248, n=n@entry=0x7f4eabbc530a queue_if_no_path)
    at structs.c:524
#2  0x00007f4eabba6993 in merge_hwe (src=0x2148c70, dst=0x211b220) at config.c:359
#3  factorize_hwtable (hw=0x214b800, n=n@entry=77) at config.c:452
#4  0x00007f4eabba7850 in load_config (file=file@entry=0x4144c8 /etc/multipath.conf) at config.c:651
#5  0x0000000000408959 in reconfigure (vecs=vecs@entry=0x2128ae0) at main.c:2078
#6  0x00000000004063c6 in child (param=0x0) at main.c:2483
#7  main (argc=<optimized out>, argv=<optimized out>) at main.c:2725
(gdb) up
#1  0x00007f4eabba879c in add_feature (f=f@entry=0x211b248, n=n@entry=0x7f4eabbc530a queue_if_no_path)
    at structs.c:524
524             if (strstr(*f, n))
(gdb) p *f
 = 0x0

Reasons:
When multipathd loads config, it will call merge_hwe() to merge the features between default_hw[] and config.
If the vender is not configured features in default_hw[], dst->features's value will be null. So strstr()'s first
parameter *f is null in add_feature(),too.

Signed-off-by: wei huang <huang.wei56@zte.com.cn>
(cherry picked from commit b1ecdd4)

libmultipath/structs.c

@@ -529,6 +529,17 @@ add_feature (char **f, char *n)
 	if (!n || *n == '0')
 		return 0;
 
+	/* default feature is null */
+	if(!*f)
+	{
+		l = asprintf(&t, "1 %s", n);
+		if(l == -1)
+			return 1;
+
+		*f = t;
+		return 0;
+	}
+
 	/* Check if feature is already present */
 	if (*f) {
 		if (strstr(*f, n))