pgsql: Fix segmentation fault in MergeInheritedAttribute()

Fix segmentation fault in MergeInheritedAttribute()

While converting a pg_attribute tuple into a ColumnDef,
ColumnDef::compression remains NULL if there is no compression method
set fot the attribute.  Calling strcmp() with NULL
ColumnDef::compression, when comparing compression methods of parents,
causes segmentation fault in MergeInheritedAttribute().  Skip
comparing compression methods if either of them is NULL.

Author: Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://www.postgresql.org/message-id/b22a6834-aacb-7b18-0424-a3f5fe889667%40gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/8f0a97dfffc5b5f48f609fb894befcdf761dd576

Modified Files
--------------
src/backend/commands/tablecmds.c          | 16 ++++++++++------
src/test/regress/expected/compression.out | 10 +++++++---
src/test/regress/sql/compression.sql      |  8 +++++---
3 files changed, 22 insertions(+), 12 deletions(-)