Fix null-pointer crash in ECPG compiler.

Fix null-pointer crash in ECPG compiler.

When compiling a DECLARE section containing a union nested
inside a struct, ecpg passes a null value for struct_sizeof to
ECPGmake_struct_type.  I (tgl) didn't foresee that case in
commit 0e6060790, and wrote an unprotected mm_strdup() call.

Reported-by: iMSA (via Jehan-Guillaume de Rorthais <jgdr@dalibo.com>)
Author: Jehan-Guillaume de Rorthais <jgdr@dalibo.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/20260625114849.34b2148e@karst
Backpatch-through: 18