pgcrypto: Remove static storage class from variables

pgcrypto: Remove static storage class from variables

Variables p, sp and ep were labeled with static storage class
but are all assigned before use so they cannot carry any data
across calls.  Fix by removing the static label.

Also while in there, make the magic variable const as it will
never change.

Author: Japin Li <japinli@hotmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/ME0P300MB0445096B67ACE8CE25772F00B6F72@ME0P300MB0445.AUSP300.PROD.OUTLOOK.COM