Dont crash on C source code (one off error)

The fix avoids one off error in case the last character in the buffer
needs to be escaped but there is not enough space in buffer to perform
the escape.

The change just simiply ignores the character in such case.

Author: Jan Stranik <jan@stranik.org>


*** /var/home/janstranik/src/emacs-27.2/lib-src/ebrowse.c~	2021-01-28 11:52:16.000000000 -0600
--- /var/home/janstranik/src/emacs-27.2/lib-src/ebrowse.c	2021-09-24 09:31:49.136287028 -0500
***************
*** 1924,1931 ****
      {
        *--s = *--t;
  
!       if (*s == '"' || *s == '\\')
!         *--s = '\\';
      }
  
    *(matching_regexp_end_buf - 1) = '\0';
--- 1924,1937 ----
      {
        *--s = *--t;
  
!       if (*s == '"' || *s == '\\') {
!           if (s > matching_regexp_buffer)
!               *--s = '\\';
!           else {
!               s++;
!               break;
!           }
!       }
      }
  
    *(matching_regexp_end_buf - 1) = '\0';