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.

src/libstrongswan/asn1/asn1.c

@@ -851,15 +851,14 @@ chunk_t asn1_bitstring(const char *mode, chunk_t content)
  */
 chunk_t asn1_integer(const char *mode, chunk_t content)
 {
-	chunk_t object;
+	chunk_t zero = chunk_from_chars(0x00), object;
 	size_t len;
 	u_char *pos;
 	bool move;
 
-
 	if (content.len == 0)
 	{	/* make sure 0 is encoded properly */
-		content = chunk_from_chars(0x00);
+		content = zero;
 		move = FALSE;
 	}
 	else