Fix GCC 7 warnings

From -Wimplicit-fallthrough which is now enabled by -W.

Source/Modules/pike.cxx

@@ -261,10 +261,9 @@ public:
       rename = strip(name);
       Printf(f_classInit, "ADD_FUNCTION(\"%s\", %s, tFunc(%s), 0);\n", rename, function, description);
       break;
-    case CLASS_CONST:
-      assert(false);		// shouldn't have gotten here for CLASS_CONST nodes
-    default:
-      assert(false);		// what is this?
+    case CLASS_CONST: // shouldn't have gotten here for CLASS_CONST nodes
+    default: // what is this?
+      assert(false);
     }
     Delete(rename);
   }

Source/Modules/ruby.cxx

@@ -1419,9 +1419,8 @@ public:
     case DESTRUCTOR:
     case CLASS_CONST:
     case STATIC_VAR:
-      assert(false);		// Should not have gotten here for these types
     default:
-      assert(false);
+      assert(false);		// Should not have gotten here for these types
     }
 
     defineAliases(n, iname);

Source/Preprocessor/cpp.c

@@ -1459,7 +1459,7 @@ String *Preprocessor_parse(String *s) {
 	break;
       }
       state = 43;
-      /* no break intended here */
+      /* FALL THRU */
 
     case 43:
       /* Get preprocessor value */