Remove use of VLA.
Remove use of VLA.

VLAs complicate static analysis and bloat stack size. Replace VLA allocation
with calls to malloc and free.  This will alos the code to build with -Wvla.

google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf