lex.c: add default, though it should never happen (re: b0282f26)

If a bug is ever introduced that causes a [[ ... ]] operator to be
unhandled by the linter, we should at least avoid writing random
memory contents to standard error. In non-release builds, let's
abort() so the problem can be more easily backtraced.

src/cmd/ksh93/sh/lex.c

@@ -1411,6 +1411,13 @@ breakloop:
 					    case TEST_GE:
 						alt = ">=";  /* '-ge' --> '>=' */
 						break;
+					    default:
+#if _AST_ksh_release
+						alt = NIL(char*);	/* output '(null)' (should never happen) */
+#else
+						abort();
+#endif
+						break;
 					}
 					errormsg(SH_DICT, ERROR_warn(0), e_lexobsolete4,
 							shp->inlineno, state, alt);