patch 9.0.1477: crash when recovering from corrupted swap file

Problem:    Crash when recovering from corrupted swap file.
Solution:   Check for a valid page count. (closes #12275)
diff --git a/src/errors.h b/src/errors.h
index f54adc7..8241dbf 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -3459,3 +3459,5 @@
 EXTERN char e_incomplete_type[]
 	INIT(= N_("E1363: Incomplete type"));
 #endif
+EXTERN char e_warning_pointer_block_corrupted[]
+	INIT(= N_("E1364: Warning: Pointer block corrupted"));