Fixes for issues found with cppcheck tool

The patch fixes small number of errors/false positives reported by `cppcheck`,
static analysis tool for C/C++.

cppcheck 1.72

$ cppcheck . --force --quiet
[cmd/zfs/zfs_main.c:4444]: (error) Possible null pointer dereference: who_perm
[cmd/zfs/zfs_main.c:4445]: (error) Possible null pointer dereference: who_perm
[cmd/zfs/zfs_main.c:4446]: (error) Possible null pointer dereference: who_perm
[cmd/zpool/zpool_iter.c:317]: (error) Uninitialized variable: nvroot
[cmd/zpool/zpool_vdev.c:1526]: (error) Memory leak: child
[lib/libefi/rdwr_efi.c:1118]: (error) Memory leak: efi_label
[lib/libuutil/uu_misc.c:207]: (error) va_list 'args' was opened but not closed by va_end().
[lib/libzfs/libzfs_import.c:1554]: (error) Dangerous usage of 'diskname' (strncpy doesn't always null-terminate it).
[lib/libzfs/libzfs_sendrecv.c:3279]: (error) Dereferencing 'cp' after it is deallocated / released
[tests/zfs-tests/cmd/file_write/file_write.c:154]: (error) Possible null pointer dereference: operation
[tests/zfs-tests/cmd/randfree_file/randfree_file.c:90]: (error) Memory leak: buf
[cmd/zinject/zinject.c:1068]: (error) Uninitialized variable: dataset
[module/icp/io/sha2_mod.c:698]: (error) Uninitialized variable: blocks_per_int64

Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1392

cmd/zfs/zfs_main.c

@@ -4553,7 +4553,7 @@ parse_fs_perm(fs_perm_t *fsperm, nvlist_t *nvl)
 				who_perm = &node->who_perm;
 			}
 		}
-
+		VERIFY3P(who_perm, !=, NULL);
 		(void) parse_who_perm(who_perm, nvl2, perm_locality);
 	}