curses: Fixes curses compiling errors.

This is the compiling error:
../ui/curses.c: In function 'curses_refresh':
../ui/curses.c:256:5: error: 'next_maybe_keycode' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  256 |     curses2foo(_curses2keycode, _curseskey2keycode, chr, maybe_keycode)
      |     ^~~~~~~~~~
../ui/curses.c:302:32: note: 'next_maybe_keycode' was declared here
  302 |             enum maybe_keycode next_maybe_keycode;
      |                                ^~~~~~~~~~~~~~~~~~
../ui/curses.c:256:5: error: 'maybe_keycode' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  256 |     curses2foo(_curses2keycode, _curseskey2keycode, chr, maybe_keycode)
      |     ^~~~~~~~~~
../ui/curses.c:265:24: note: 'maybe_keycode' was declared here
  265 |     enum maybe_keycode maybe_keycode;
      |                        ^~~~~~~~~~~~~
cc1.exe: all warnings being treated as errors

gcc version 10.2.0 (Rev1, Built by MSYS2 project)

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20201012234348.1427-4-luoyonggang@gmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

ui/curses.c

@@ -262,7 +262,7 @@ static int curses2foo(const int _curses2foo[], const int _curseskey2foo[],
 static void curses_refresh(DisplayChangeListener *dcl)
 {
     int chr, keysym, keycode, keycode_alt;
-    enum maybe_keycode maybe_keycode;
+    enum maybe_keycode maybe_keycode = CURSES_KEYCODE;
 
     curses_winch_check();
 
@@ -299,7 +299,7 @@ static void curses_refresh(DisplayChangeListener *dcl)
 
         /* alt or esc key */
         if (keycode == 1) {
-            enum maybe_keycode next_maybe_keycode;
+            enum maybe_keycode next_maybe_keycode = CURSES_KEYCODE;
             int nextchr = console_getch(&next_maybe_keycode);
 
             if (nextchr != -1) {