Panic when parsing "(?m){1,1}"

[adamcrume]

Parsing "(?m){1,1}" with regex-syntax 0.6.5 results in:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:355:21                                                                       
stack backtrace:                           
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace                       
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49                     
   1: std::sys_common::backtrace::_print   
             at src/libstd/sys_common/backtrace.rs:71                                 
   2: std::panicking::default_hook::{{closure}}                                       
             at src/libstd/sys_common/backtrace.rs:59                                 
             at src/libstd/panicking.rs:211                                           
   3: std::panicking::default_hook         
             at src/libstd/panicking.rs:227
   4: std::panicking::rust_panic_with_hook 
             at src/libstd/panicking.rs:491
   5: std::panicking::continue_panic_fmt   
             at src/libstd/panicking.rs:398                                           
   6: rust_begin_unwind                    
             at src/libstd/panicking.rs:325
   7: core::panicking::panic_fmt           
             at src/libcore/panicking.rs:95                                                                                                                                 
   8: core::panicking::panic               
             at src/libcore/panicking.rs:59
   9: <core::option::Option<T>>::unwrap
             at /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libcore/macros.rs:20
  10: <regex_syntax::hir::translate::TranslatorI<'t, 'p> as regex_syntax::ast::visitor::Visitor>::visit_post
             at /home/adam/.cargo/registry/src/github.lhy31512.workers.dev-1ecc6299db9ec823/regex-syntax-0.6.5/src/hir/translate.rs:339
  11: regex_syntax::ast::visitor::HeapVisitor::visit
             at /home/adam/.cargo/registry/src/github.lhy31512.workers.dev-1ecc6299db9ec823/regex-syntax-0.6.5/src/ast/visitor.rs:261
  12: regex_syntax::ast::visitor::visit
             at /home/adam/.cargo/registry/src/github.lhy31512.workers.dev-1ecc6299db9ec823/regex-syntax-0.6.5/src/ast/visitor.rs:130
  13: regex_syntax::hir::translate::Translator::translate
             at /home/adam/.cargo/registry/src/github.lhy31512.workers.dev-1ecc6299db9ec823/regex-syntax-0.6.5/src/hir/translate.rs:143
  14: regex_syntax::parser::Parser::parse
             at /home/adam/.cargo/registry/src/github.lhy31512.workers.dev-1ecc6299db9ec823/regex-syntax-0.6.5/src/parser.rs:203