libpq code should use libpq_gettext(), not _()

Fix some wrong use and install a safeguard against future mistakes.

Author: petere

src/interfaces/libpq/fe-auth-scram.c

@@ -922,7 +922,7 @@ pg_fe_scram_build_secret(const char *password, const char **errstr)
 	rc = pg_saslprep(password, &prep_password);
 	if (rc == SASLPREP_OOM)
 	{
-		*errstr = _("out of memory");
+		*errstr = libpq_gettext("out of memory");
 		return NULL;
 	}
 	if (rc == SASLPREP_SUCCESS)
@@ -931,7 +931,7 @@ pg_fe_scram_build_secret(const char *password, const char **errstr)
 	/* Generate a random salt */
 	if (!pg_strong_random(saltbuf, SCRAM_DEFAULT_SALT_LEN))
 	{
-		*errstr = _("could not generate random salt");
+		*errstr = libpq_gettext("could not generate random salt");
 		free(prep_password);
 		return NULL;
 	}

src/interfaces/libpq/libpq-int.h

@@ -881,6 +881,11 @@ extern char *libpq_ngettext(const char *msgid, const char *msgid_plural, unsigne
 #define libpq_gettext(x) (x)
 #define libpq_ngettext(s, p, n) ((n) == 1 ? (s) : (p))
 #endif
+/*
+ * libpq code should use the above, not _(), since that would use the
+ * surrounding programs's message catalog.
+ */
+#undef _
 
 /*
  * These macros are needed to let error-handling code be portable between