fix ecpg core dump when there's a very long struct variable name in .pgc file

hi

I found a small bug in ecpg command and try to fix it.
Please check if it is correct.

When use a struct variable whose name length is very very long such as 12KB in .pgc source,
ecpg will core dump because of buffer overflow if precompile the .pgc file.

$ ecpg testLongStructName.pgc
Segmentation fault (core dumped)

Normally no body will write a variable with so long name,
but whether it's better to fix it.

Best Regards，
Chen Huajun