Clean up error message reported after \password encryption failure.

Experimenting with FIPS mode enabled, I saw

regression=# \password joe
Enter new password for user "joe":
Enter it again:
could not encrypt password: disabled for FIPS
out of memory

because PQencryptPasswordConn was still of the opinion that "out of
memory" is always appropriate to print.

Minor oversight in b69aba745.  Like that one, back-patch to v14.

src/interfaces/libpq/fe-auth.c

@@ -1265,6 +1265,10 @@ PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user,
 	if (strcmp(algorithm, "scram-sha-256") == 0)
 	{
 		crypt_pwd = pg_fe_scram_build_secret(passwd);
+		/* We assume the only possible failure is OOM */
+		if (!crypt_pwd)
+			appendPQExpBufferStr(&conn->errorMessage,
+								 libpq_gettext("out of memory\n"));
 	}
 	else if (strcmp(algorithm, "md5") == 0)
 	{
@@ -1282,6 +1286,9 @@ PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user,
 				crypt_pwd = NULL;
 			}
 		}
+		else
+			appendPQExpBufferStr(&conn->errorMessage,
+								 libpq_gettext("out of memory\n"));
 	}
 	else
 	{
@@ -1291,9 +1298,5 @@ PQencryptPasswordConn(PGconn *conn, const char *passwd, const char *user,
 		return NULL;
 	}
 
-	if (!crypt_pwd)
-		appendPQExpBufferStr(&conn->errorMessage,
-							 libpq_gettext("out of memory\n"));
-
 	return crypt_pwd;
 }