Fix multiplex backreferencs near end of string in regexp match

Idea from Jirka Marsik.

Fixes [Bug #18631]

regexec.c

@@ -1178,11 +1178,13 @@ static int string_cmp_ic(OnigEncoding enc, int case_fold_flag,
 # define DATA_ENSURE_CHECK1    (s < right_range)
 # define DATA_ENSURE_CHECK(n)  (s + (n) <= right_range)
 # define DATA_ENSURE(n)        if (s + (n) > right_range) goto fail
+# define DATA_ENSURE_CONTINUE(n) if (s + (n) > right_range) continue
 # define ABSENT_END_POS        right_range
 #else
 # define DATA_ENSURE_CHECK1    (s < end)
 # define DATA_ENSURE_CHECK(n)  (s + (n) <= end)
 # define DATA_ENSURE(n)        if (s + (n) > end) goto fail
+# define DATA_ENSURE_CONTINUE(n) if (s + (n) > end) continue
 # define ABSENT_END_POS        end
 #endif /* USE_MATCH_RANGE_MUST_BE_INSIDE_OF_SPECIFIED_RANGE */
 
@@ -2645,7 +2647,7 @@ match_at(regex_t* reg, const UChar* str, const UChar* end,
 		  ? STACK_AT(mem_end_stk[mem])->u.mem.pstr
 		  : (UChar* )((void* )mem_end_stk[mem]));
 	  n = pend - pstart;
-	  DATA_ENSURE(n);
+	  DATA_ENSURE_CONTINUE(n);
 	  sprev = s;
 	  swork = s;
 	  STRING_CMP_VALUE(pstart, swork, n, is_fail);
@@ -2684,7 +2686,7 @@ match_at(regex_t* reg, const UChar* str, const UChar* end,
 		  ? STACK_AT(mem_end_stk[mem])->u.mem.pstr
 		  : (UChar* )((void* )mem_end_stk[mem]));
 	  n = pend - pstart;
-	  DATA_ENSURE(n);
+	  DATA_ENSURE_CONTINUE(n);
 	  sprev = s;
 	  swork = s;
 	  STRING_CMP_VALUE_IC(case_fold_flag, pstart, &swork, n, end, is_fail);

test/ruby/test_regexp.rb

@@ -1422,6 +1422,12 @@ class TestRegexp < Test::Unit::TestCase
     end
   end
 
+  def test_bug18631
+    assert_kind_of MatchData, /(?<x>a)(?<x>aa)\k<x>/.match("aaaaa")
+    assert_kind_of MatchData, /(?<x>a)(?<x>aa)\k<x>/.match("aaaa")
+    assert_kind_of MatchData, /(?<x>a)(?<x>aa)\k<x>/.match("aaaab")
+  end
+
   # This assertion is for porting x2() tests in testpy.py of Onigmo.
   def assert_match_at(re, str, positions, msg = nil)
     re = Regexp.new(re) unless re.is_a?(Regexp)