asn1: Fix a compiler warning with GCC 9.1

asn1: Fix a compiler warning with GCC 9.1

Compiling with GCC 9.1, as e.g. happens on AppVeyor, results in the
following warning:

asn1/asn1.c: In function 'asn1_integer':
asn1/asn1.c:871:24: error: '<Ucb40>' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  871 |  len = content.len + ((*content.ptr & 0x80) ? 1 : 0);
      |                        ^~~~~~~~~~~~

Some experiments showed that the problem was the chunk_from_chars()
assignment.  This might be because the temporary chunk_t that was assigned
to the variable was defined in a sub-block, so it might actually be
undefined later when *content.ptr is read.