Off-by-one error in windows-nat.c causes abort at startup

[Eli Zaretskii <eliz at gnu dot org>]

I created a gdb.ini file in my home directory, and suddenly found that
almost all my GDB binaries stopped working.  Even "gdb --version"
would crash at startup thusly:

  ./common/common-utils.c:141: internal-error: xsnprintf: Assertion `ret < size' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Quit this debugging session? (y or n) [answered Y; input not from terminal]

  This is a bug, please report it.  For instructions, see:
  <http://www.gnu.org/software/gdb/bugs/>.

  ./common/common-utils.c:141: internal-error: xsnprintf: Assertion `ret < size' failed.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Create a core file of GDB? (y or n) [answered Y; input not from terminal]

  This application has requested the Runtime to terminate it in an unusual way.
  Please contact the application's support team for more information.

Luckily, I still had GDB 7.5, which did work.  Using it, I found the
off-by-one gotcha below (".gdbinit" is one character longer than
"gdb.ini").  I guess no one tested this feature when we switched from
using snprintf to xsnprintf...

OK to commit (with a suitable ChangeLog entry, of course)?

--- gdb/windows-nat.c~	2016-02-10 05:19:39.000000000 +0200
+++ gdb/windows-nat.c	2016-04-30 11:57:08.500000000 +0300
@@ -2711,9 +2711,9 @@ _initialize_check_for_gdb_ini (void)
       if (access (oldini, 0) == 0)
 	{
 	  int len = strlen (oldini);
-	  char *newini = (char *) alloca (len + 1);
+	  char *newini = (char *) alloca (len + 2);
 
-	  xsnprintf (newini, len + 1, "%.*s.gdbinit",
+	  xsnprintf (newini, len + 2, "%.*s.gdbinit",
 		     (int) (len - (sizeof ("gdb.ini") - 1)), oldini);
 	  warning (_("obsolete '%s' found. Rename to '%s'."), oldini, newini);
 	}