bug#23949: 25.0.95; Regression in handling error caused by (string-match-p "." nil)

[Eli Zaretskii]

> From: Kaushal Modi <kaushal.modi@gmail.com>
> Date: Tue, 12 Jul 2016 12:29:25 +0000
> 
> Lisp Backtrace:
> "read-from-string" (0xffff3a30)
> "help-function-arglist" (0xffff3f88)

This says it tried to read from a string.  What string was that?  And
why does help-function-arglist try to read from that string?

> #9 0x0000000000623fab in Fread_from_string (string=57797668, start=0, end=0) 
> at lread.c:2075

In this frame, please show the value of 'string', like this:

 (gdb) frame 9
 (gdb) p string
 (gdb) xstring