state: Ignore unassigned keys in start and select states

That would cause incompatible behavior when we switched to
libxkbcommon to resolve keysyms:
#54 (comment)

libskk/state.vala

@@ -745,6 +745,11 @@ namespace Skk {
                                                   ref KeyEvent key)
         {
             var command = state.lookup_key (key);
+
+            // if no command nor code is assigned to key, we can't proceed
+            if (command == null && key.code == 0)
+                return true;
+
             if (command == "abort" ||
                 command == "abort-to-latin" ||
                 command == "abort-to-latin-unhandled") {
@@ -976,6 +981,11 @@ namespace Skk {
                                                   ref KeyEvent key)
         {
             var command = state.lookup_key (key);
+
+            // if no command nor code is assigned to key, we can't proceed
+            if (command == null && key.code == 0)
+                return true;
+
             if (command == "previous-candidate") {
                 if (!state.candidates.previous ()) {
                     state.candidates.clear ();

tests/basic.c

@@ -57,6 +57,7 @@ static SkkTransition rom_kana_transitions[] =
     { SKK_INPUT_MODE_LATIN, "a \t", "", "a", SKK_INPUT_MODE_LATIN },
     /* Issue#11 */
     { SKK_INPUT_MODE_HIRAGANA, "q s a n S y a", "▽シャ", "サン", SKK_INPUT_MODE_KATAKANA },
+    { SKK_INPUT_MODE_HIRAGANA, "H o h Control_L a a a a a", "▽ほはああああ", "", SKK_INPUT_MODE_HIRAGANA },
     { 0, NULL }
   };
 
@@ -78,6 +79,7 @@ static SkkTransition okuri_nasi_transitions[] =
     { SKK_INPUT_MODE_KATAKANA, "K a n j i SPC q", "", "漢字", SKK_INPUT_MODE_HIRAGANA },
     { SKK_INPUT_MODE_HIRAGANA, "N A N A", "▼な*んあ【】", "", SKK_INPUT_MODE_HIRAGANA },
     { SKK_INPUT_MODE_HIRAGANA, "N A N a", "▼な*な【】", "", SKK_INPUT_MODE_HIRAGANA },
+    { SKK_INPUT_MODE_HIRAGANA, "A o SPC Control_L", "▼青", "", SKK_INPUT_MODE_HIRAGANA },
     { 0, NULL }
   };