Fix: Dereference null return value

Fix: Dereference null return value

Found by Coverity:

3. returned_null: bt_ctf_field_string_get_value returns null (checked 10
out of 12 times).

CID 1376192 (#1 of 1): Dereference null return value (NULL_RETURNS)
4. dereference: Dereferencing a pointer that might be null
bt_ctf_field_string_get_value(field) when calling print_escape_string.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>