fix[codegen]: fix abi_encode buffer size in external calls (#4202)

fix the buffer size passed to the abi encoder from the external call
argument packer. previously, the size was overestimated as it didn't
take into account pointer arithmetic performed on the buffer passed to
abi_encode.

this is a hygienic fix, no security implications were found for this
issue.

---------

Co-authored-by: Charles Cooper <cooper.charles.m@gmail.com>

Author: cyberthirst

vyper/codegen/external_call.py

@@ -71,7 +71,9 @@ def _pack_arguments(fn_type, args, context):
     pack_args.append(["mstore", buf, util.method_id_int(abi_signature)])
 
     if len(args) != 0:
-        pack_args.append(abi_encode(add_ofst(buf, 32), args_as_tuple, context, bufsz=buflen))
+        encode_buf = add_ofst(buf, 32)
+        encode_buflen = buflen - 32
+        pack_args.append(abi_encode(encode_buf, args_as_tuple, context, bufsz=encode_buflen))
 
     return buf, pack_args, args_ofst, args_len