[ruby/yarp] fix: trailing asterisk at end of file

Previously this resulted in invalid memory access.

Found by the fuzzer.

https://github.com/ruby/yarp/commit/c86b4907b4

test/yarp/fuzzer_test.rb

@@ -16,4 +16,5 @@ class FuzzerTest < Test::Unit::TestCase
   snippet "incomplete symbol", ":"
   snippet "incomplete escaped string", '"\\'
   snippet "trailing comment", "1\n#\n"
+  snippet "trailing asterisk", "a *"
 end

yarp/yarp.c

@@ -380,6 +380,9 @@ lex_state_arg_p(yp_parser_t *parser) {
 
 static inline bool
 lex_state_spcarg_p(yp_parser_t *parser, bool space_seen) {
+    if (parser->current.end >= parser->end) {
+        return false;
+    }
     return lex_state_arg_p(parser) && space_seen && !yp_char_is_whitespace(*parser->current.end);
 }