[ruby/yarp] fix: string escape char "\" at the end of a file

Previously this resulted in invalid memory access.

Found by the fuzzer.

https://github.com/ruby/yarp/commit/178862e2ca

test/yarp/fuzzer_test.rb

@@ -14,4 +14,5 @@ class FuzzerTest < Test::Unit::TestCase
 
   snippet "incomplete global variable", "$"
   snippet "incomplete symbol", ":"
+  snippet "incomplete escaped string", '"\\'
 end

yarp/yarp.c

@@ -7224,6 +7224,12 @@ parser_lex(yp_parser_t *parser) {
                         breakpoint = yp_strpbrk(parser, breakpoint + 1, breakpoints, parser->end - (breakpoint + 1));
                         break;
                     case '\\': {
+                        // Check that we're not at the end of the file.
+                        if (breakpoint + 1 >= parser->end) {
+                            breakpoint = NULL;
+                            break;
+                        }
+
                         // If we hit escapes, then we need to treat the next token
                         // literally. In this case we'll skip past the next character and
                         // find the next breakpoint.