[bgpd] review 32-bit AS-path hotfix for 0.99.12

[bgpd] review 32-bit AS-path hotfix for 0.99.12

The patch by Chris Caputo, which was used to prepare 0.99.12
release, consists of three parts:

1. memory allocation fix itself
2. fix for warnings about constant variables
3. fix for printf format specs (%d was used instead of %u)

It was confirmed later, that:
a. a much simpler bugfix was available for memory allocation
b. committed version of the bugfix wasn't optimal CPU-wise

At this point I consider reasonable to revert the allocation
portion of that patch and to replace it with the shorter
version, which is:

-#define ASN_STR_LEN (5 + 1)
+#define ASN_STR_LEN (10 + 1)

Other two parts of Mr. Caputo's patch remain intact.