storage: Fix daemon crash on lookup storagepool by targetpath

Causing a crash when storagePoolLookupByTargetPath beacuse of
Some types of storage pool have no target elements.
Use STREQ_NULLABLE instead of STREQ
Avoids segfaults when using NULL arguments.

Core was generated by `/usr/sbin/libvirtd'.
Program terminated with signal 11, Segmentation fault.
(gdb) bt
0  0x0000ffff9e951388 in strcmp () from /lib64/libc.so.6
1  0x0000ffff92103e9c in storagePoolLookupByTargetPathCallback (
    obj=0xffff7009aab0, opaque=0xffff801058b0) at storage/storage_driver.c:1649
2  0x0000ffff9f2c52a4 in virStoragePoolObjListSearchCb (
    payload=0xffff801058b0, name=<optimized out>, opaque=<optimized out>)
    at conf/virstorageobj.c:476
3  0x0000ffff9f1f2f7c in virHashSearch (ctable=0xffff800f4f60,
    iter=iter@entry=0xffff9f2c5278 <virStoragePoolObjListSearchCb>,
    data=data@entry=0xffff95af7488, name=name@entry=0x0) at util/virhash.c:696
4  0x0000ffff9f2c64f0 in virStoragePoolObjListSearch (pools=0xffff800f2ce0,
    searcher=searcher@entry=0xffff92103e68 <storagePoolLookupByTargetPathCallback>,
     opaque=<optimized out>) at conf/virstorageobj.c:505
5  0x0000ffff92101f54 in storagePoolLookupByTargetPath (conn=0xffff5c0009f0,
path=0xffff7009a850 "/vms/images") at storage/storage_driver.c:1672

Reviewed-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Yi Li <yili@winhong.com>

src/storage/storage_driver.c

@@ -1713,7 +1713,7 @@ storagePoolLookupByTargetPathCallback(virStoragePoolObjPtr obj,
         return false;
 
     def = virStoragePoolObjGetDef(obj);
-    return STREQ(path, def->target.path);
+    return STREQ_NULLABLE(path, def->target.path);
 }