Loop when necessary in contrib/pgcrypto's pktreader_pull().

This fixes a scenario in which pgp_sym_decrypt() failed with "Wrong key
or corrupt data" on messages whose length is 6 less than a power of 2.

Per bug #11905 from Connor Penhale.  Fix by Marko Tiikkaja, regression
test case from Jeff Janes.

contrib/pgcrypto/expected/pgp-decrypt.out

@@ -364,3 +364,11 @@ a3nsOzKTXUfS9VyaXo8IrncM6n7fdaXpwba/3tNsAhJG4lDv1k4g9v8Ix2dfv6Rs
 (1 row)
 
 -- expected: 7efefcab38467f7484d6fa43dc86cf5281bd78e2
+-- check BUG #11905, problem with messages 6 less than a power of 2.
+select pgp_sym_decrypt(pgp_sym_encrypt(repeat('x',65530),'1'),'1') = repeat('x',65530);
+ ?column? 
+----------
+ t
+(1 row)
+
+-- expected: true

contrib/pgcrypto/pgp-decrypt.c

@@ -182,7 +182,7 @@ pktreader_pull(void *priv, PullFilter *src, int len,
 	if (pkt->type == PKT_CONTEXT)
 		return pullf_read(src, len, data_p);
 
-	if (pkt->len == 0)
+	while (pkt->len == 0)
 	{
 		/* this was last chunk in stream */
 		if (pkt->type == PKT_NORMAL)

contrib/pgcrypto/sql/pgp-decrypt.sql

@@ -264,3 +264,7 @@ a3nsOzKTXUfS9VyaXo8IrncM6n7fdaXpwba/3tNsAhJG4lDv1k4g9v8Ix2dfv6Rs
 -----END PGP MESSAGE-----
 '), 'key', 'convert-crlf=1'), 'sha1'), 'hex');
 -- expected: 7efefcab38467f7484d6fa43dc86cf5281bd78e2
+
+-- check BUG #11905, problem with messages 6 less than a power of 2.
+select pgp_sym_decrypt(pgp_sym_encrypt(repeat('x',65530),'1'),'1') = repeat('x',65530);
+-- expected: true