_asn1_expand_object_id uses null pointer as string

[Pascal Cuoq]

Here is a minor issue with no security consequences (the description file is 
not usually under the control of an attacker).

With the specially crafted ASN.1 description attached to this message, using 
the current git version, the function _asn1_expand_object_id, at line 797, 
passes a null pointer as p4->value to the function _asn1_str_cat, which expects 
a pointer to a string: 
http://git.savannah.gnu.org/cgit/libtasn1.git/tree/lib/parser_aux.c?id=a6e0a0b58f5cdaf4e9beca5bce69c09808cbb625#n797

The callstack at the point of the crash is:

stack: strlen :: lib/gstr.c:34 <-
        _asn1_str_cat :: lib/parser_aux.c:797 <-
        _asn1_expand_object_id :: ASN1.y:704 <-
        asn1_parser2tree :: src/asn1Decoding.c:155 <-
        main

The commandline to reproduce is:

src/asn1Decoding null_string.asn null_string.asn PKIX1.Certificate

null_string.asn
Description: null_string.asn

ATT00001.txt
Description: ATT00001.txt