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 
Reported-by: Alexander Lakhin 
Discussion: https://www.postgresql.org/message-id/b22a6834-aacb-7b18-0424-a3f5fe889667%40gmail.com