bug#26047: 26.0.50; emacs crash by cl-caff2

[Leo Liu]

1. Compile the following t.el file:

(require 'cl-lib)

(put 's 'buf "abcd")
(cl-callf2 cl-concatenate 'string (get 's 'buf) "efg")

(provide 't)

2. Load the compiled t.elc file

Crash on centos7:

Fatal error 11: Segmentation fault
Backtrace:
emacs[0x80e40be]
emacs[0x80ccbfd]
emacs[0x80e2c67]
emacs[0x80e2ebe]
emacs[0x80e2ee6]
[0xf77e0410]
emacs[0x813a17a]
emacs[0x816bffe]
emacs[0x816ddc7]
emacs[0x8139908]
emacs[0x815afc8]
emacs[0x815b42e]
emacs[0x813b1f4]
emacs[0x813a2ac]
emacs[0x816bffe]
emacs[0x8139ebb]
emacs[0x813a22b]
emacs[0x816bffe]
emacs[0x8139ebb]
emacs[0x813a22b]
emacs[0x816bffe]
emacs[0x8139ebb]
emacs[0x813a22b]
emacs[0x81364fb]
emacs[0x813a2ac]
emacs[0x8136e9f]
emacs[0x813a2ac]
emacs[0x816bffe]
emacs[0x8139fdd]
emacs[0x813a22b]
emacs[0x813a387]
emacs[0x80d9b72]
emacs[0x8138afe]
emacs[0x80ccfa4]
emacs[0x8138abb]
emacs[0x80ccf61]
emacs[0x80d116e]
emacs[0x80d146c]
emacs[0x804cc03]
/lib/libc.so.6(__libc_start_main+0xf3)[0xf75869a3]
emacs[0x804db03]
...
Segmentation fault