[ruby/yarp] fix: ":" at the end of a file

Previously this resulted in invalid memory access.

Found by the fuzzer.

https://github.com/ruby/yarp/commit/c781c9fcd2

test/yarp/fuzzer_test.rb

@@ -13,4 +13,5 @@ class FuzzerTest < Test::Unit::TestCase
   end
 
   snippet "incomplete global variable", "$"
+  snippet "incomplete symbol", ":"
 end

yarp/yarp.c

@@ -6581,7 +6581,7 @@ parser_lex(yp_parser_t *parser) {
                         LEX(YP_TOKEN_COLON_COLON);
                     }
 
-                    if (lex_state_end_p(parser) || yp_char_is_whitespace(*parser->current.end) || peek(parser) == '#') {
+                    if (lex_state_end_p(parser) || yp_char_is_whitespace(peek(parser)) || peek(parser) == '#') {
                         lex_state_set(parser, YP_LEX_STATE_BEG);
                         LEX(YP_TOKEN_COLON);
                     }