patch for less v97: unset variable in exec_mca() can lead to core dump

[Paul Eggert]

In less version 97, a local variable 'n' in exec_mca() is not initialized.
If n's initial garbage is a small negative number, 'less' can dump core because
it allocates a buffer that is too small, and then overruns the buffer.
Here's a fix to line 309 of command.c.

*** less97/command.c	Mon Sep 26 09:03:50 1988
--- patched/command.c	Mon Sep 26 08:48:26 1988
***************
*** 307,311 ****
  			 * need to allocate for the expanded shell cmd.
  			 */
! 			for (fr = cmdbuf;  *fr != '\0';  fr++)
  				if (*fr == '%')
  					n += strlen(current_file);
--- 307,311 ----
  			 * need to allocate for the expanded shell cmd.
  			 */
! 			for (fr = cmdbuf, n = 0;  *fr != '\0';  fr++)
  				if (*fr == '%')
  					n += strlen(current_file);