Add missing connection validation in ECPG

Add missing connection validation in ECPG

ECPGdeallocate_all(), ECPGprepared_statement(), ECPGget_desc(), and
ecpg_freeStmtCacheEntry() could crash with a SIGSEGV when called
without an established connection (for example, when EXEC SQL CONNECT
was forgotten or a non-existent connection name was used), because
they dereferenced the result of ecpg_get_connection() without first
checking it for NULL.

Each site is fixed in the style of the surrounding code.

New tests are added for these conditions.

Author: Shruthi Gowda <gowdashru@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Mahendra Singh Thalor <mahi6run@gmail.com>
Reviewed-by: Nishant Sharma <nishant.sharma@enterprisedb.com>
Discussion: https://postgr.es/m/3007317.1765210195@sss.pgh.pa.us
Backpatch-through: 14