pem: reject indefinite-length DER in pem_read_asn1_length() - #2702

pem_read_asn1_length() in src/pem.c takes a 0x80 length octet as the long form with zero length bytes, so the 1 + lenlen > datalen guard from #2028 passes with lenlen 0 and the next line still reads data[1], one byte past the element, and uses it as the length. On the libgcrypt backend a PEM RSA or DSA private key whose DER body is just 30 80 reaches it through libssh2_userauth_publickey_fromfile() or frommemory(), and whether the SEQUENCE is accepted depends on whatever byte happens to follow the span (AddressSanitizer reports a one byte heap-buffer-overflow read on an exactly sized buffer). 0x80 is the DER indefinite form, which never appears in a valid key, so reject it before touching the length bytes; I checked #2028 and #2011 first and this is the one long-form case that fix leaves open. The new test in tests/test_simple.c fails on master and passes with this change.