installer: Remove key_reader variable-length array

Do not use variable-length arrays.  Use an appropriately named ('k'
followed by CamelCase) compile-time constant for the size.
[runtime/arrays] [1]

BUG=b:169261043
TEST=FEATURES=test emerge-$B chromeos-installer

Change-Id: Iafd98d2ff48e77ba80776b9894632b7deef9bc8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/2585663
Tested-by: Jae Hoon Kim <kimjae@chromium.org>
Reviewed-by: Vyshu Khota <vyshu@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
diff --git a/installer/util/key_reader.cc b/installer/util/key_reader.cc
index 5f7d6a8..c550d34 100644
--- a/installer/util/key_reader.cc
+++ b/installer/util/key_reader.cc
@@ -11,6 +11,7 @@
 #include <unistd.h>
 
 #include <utility>
+#include <vector>
 
 #include <base/files/file_enumerator.h>
 #include <base/files/scoped_file.h>
@@ -183,8 +184,8 @@
 
     // Put char representation in buffer.
     int size = xkb_state_key_get_utf8(state_, keycode, nullptr, 0) + 1;
-    char buff[size];
-    xkb_state_key_get_utf8(state_, keycode, buff, size);
+    std::vector<char> buff(size);
+    xkb_state_key_get_utf8(state_, keycode, buff.data(), size);
 
     if (sym == XKB_KEY_BackSpace && !user_input_.empty()) {
       user_input_.pop_back();