Fix truncation warnings in posix/tst-glob_symlinks.c.

The test posix/tst-glob_symlinks.c fails to build with GCC mainline:

tst-glob_symlinks.c: In function 'do_test':
tst-glob_symlinks.c:124:30: error: 'snprintf' output may be truncated before the last format character [-Werror=format-truncation=]
   snprintf (buf, sizeof buf, "%s?", dangling_link);
                              ^~~~~
tst-glob_symlinks.c:124:3: note: 'snprintf' output between 2 and 4097 bytes into a destination of size 4096
   snprintf (buf, sizeof buf, "%s?", dangling_link);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tst-glob_symlinks.c:128:30: error: 'snprintf' output may be truncated before the last format character [-Werror=format-truncation=]
   snprintf (buf, sizeof buf, "%s*", dangling_link);
                              ^~~~~
tst-glob_symlinks.c:128:3: note: 'snprintf' output between 2 and 4097 bytes into a destination of size 4096
   snprintf (buf, sizeof buf, "%s*", dangling_link);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This patch fixes the test to avoid such truncation warnings by
increasing the buffer in question by one byte, to ensure it can hold
any possible result of %s? or %s* formats where %s comes from a buffer
of size PATH_MAX.

Tested compilation with build-many-glibcs.py for aarch64-linux-gnu.

	* posix/tst-glob_symlinks.c (do_test): Increase size of buf.

2 files changed, 3 insertions, 1 deletions

diff --git a/ChangeLog b/ChangeLog
index f90ddb5..a444183 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,7 @@
 2017-12-18  Joseph Myers  <joseph@codesourcery.com>
 
+	* posix/tst-glob_symlinks.c (do_test): Increase size of buf.
+
 	* string/tester.c (test_strncat): Also disable -Warray-bounds
 	warnings for two tests.
 
diff --git a/posix/tst-glob_symlinks.c b/posix/tst-glob_symlinks.c
index 5c4b4ec..3a86f4c 100644
--- a/posix/tst-glob_symlinks.c
+++ b/posix/tst-glob_symlinks.c
@@ -94,7 +94,7 @@ do_prepare (int argc, char *argv[])
 static int
 do_test (void)
 {
-  char buf[PATH_MAX];
+  char buf[PATH_MAX + 1];
   glob_t gl;
 
   TEST_VERIFY_EXIT (glob (valid_link, 0, NULL, &gl) == 0);