[ruby/prism] Fix use of uninitialized value when parsing regexp

Parsing the regexp /\A{/ causes uses an uninitialized value because it
tries to parse it as a range quantifier, so it reads the character after
the closing curly bracket. This is using uninitialized values because
prism strings are not null terminated. This can be seen in the Valgrind
output:

    ==834710== Conditional jump or move depends on uninitialised value(s)
    ==834710==    at 0x5DA010: pm_regexp_parse_range_quantifier (regexp.c:163)
    ==834710==    by 0x5DA010: pm_regexp_parse_quantifier (regexp.c:243)
    ==834710==    by 0x5DAD69: pm_regexp_parse_expression (regexp.c:738)
    ==834710==    by 0x5DAD69: pm_regexp_parse_pattern (regexp.c:761)
    ==834710==    by 0x5DAD69: pm_regexp_parse (regexp.c:773)
    ==834710==    by 0x5A2EE7: parse_regular_expression_named_captures (prism.c:20886)
    ==834710==    by 0x5A2EE7: parse_expression_infix (prism.c:21388)
    ==834710==    by 0x5A5FA5: parse_expression (prism.c:21804)
    ==834710==    by 0x5A64F3: parse_statements (prism.c:13858)
    ==834710==    by 0x5A9730: parse_program (prism.c:22011)
    ==834710==    by 0x576F0D: parse_input_success_p (extension.c:1062)
    ==834710==    by 0x576F0D: parse_success_p (extension.c:1084)

This commit adds checks for the end of the string to
pm_regexp_parse_range_quantifier.

https://github.com/ruby/prism/commit/be6cbc23ef

prism/regexp.c

@@ -158,6 +158,11 @@ pm_regexp_parse_range_quantifier(pm_regexp_parser_t *parser) {
     } state = PM_REGEXP_RANGE_QUANTIFIER_STATE_START;
 
     while (1) {
+        if (parser->cursor >= parser->end) {
+            parser->cursor = savepoint;
+            return true;
+        }
+
         switch (state) {
             case PM_REGEXP_RANGE_QUANTIFIER_STATE_START:
                 switch (*parser->cursor) {

test/prism/regexp_test.rb

@@ -186,6 +186,10 @@ module Prism
       assert_valid_regexp("foo{1, 2}")
     end
 
+    def test_fake_range_quantifier_because_unclosed
+      assert_valid_regexp("\\A{")
+    end
+
     ############################################################################
     # These test that flag values are correct.
     ############################################################################