patch 8.2.3855: illegal memory access when displaying a blob

Problem:    Illegal memory access when displaying a blob.
Solution:   Append a NUL at the end. (Yegappan Lakshmanan, closes #9372)
diff --git a/src/regexp_nfa.c b/src/regexp_nfa.c
index 9dc246d..4d7d553 100644
--- a/src/regexp_nfa.c
+++ b/src/regexp_nfa.c
@@ -2917,20 +2917,20 @@
 	ga_concat(indent, (char_u *)"| ");
     else
 	ga_concat(indent, (char_u *)"  ");
-    ga_append(indent, '\0');
+    ga_append(indent, NUL);
 
     nfa_print_state2(debugf, state->out, indent);
 
     // replace last part of indent for state->out1
     indent->ga_len -= 3;
     ga_concat(indent, (char_u *)"  ");
-    ga_append(indent, '\0');
+    ga_append(indent, NUL);
 
     nfa_print_state2(debugf, state->out1, indent);
 
     // shrink indent
     indent->ga_len -= 3;
-    ga_append(indent, '\0');
+    ga_append(indent, NUL);
 }
 
 /*