preproc.c: Fix NULL deref on token pasting

In case if there is a whitespace before
'paste' token we may reach NULL dereference
in strlen since paste_head will point to
TOK_WHITESPACE. Fix it.

[mainstream adabc1576b]

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

preproc.c

@@ -3594,7 +3594,9 @@ static bool paste_tokens(Token **head, bool handle_paste_tokens)
             }
             /* else fall through */
         default:
-            tail = paste_head = &t->next;
+            tail = &t->next;
+            if (!tok_type_(t->next, TOK_WHITESPACE))
+                paste_head = tail;
             break;
         }
     }