jruby: Check if len++ walked off the end (#153)

Fix GH-152

CRuby can walk off the end because there's always a null byte. In JRuby,
the byte array is often (usually?) the exact size of the string. So we
need to check if len++ walked off the end.

This code was ported from a version by @byroot in
#127 but I missed adding this check
due to a lack of tests. A test is included for both "-" and "+" parsing.

Author: headius

ext/jruby/org/jruby/ext/strscan/RubyStringScanner.java

@@ -589,7 +589,7 @@ public IRubyObject scan_base10_integer(ThreadContext context) {
             len++;
         }
 
-        if (!Character.isDigit(bytes.get(ptr + len))) {
+        if (len >= remaining_len || !Character.isDigit(bytes.get(ptr + len))) {
             return context.nil;
         }
 

test/strscan/test_stringscanner.rb

@@ -1001,6 +1001,16 @@ def test_scan_integer
     assert_equal(0, s.pos)
     refute_predicate(s, :matched?)
 
+    s = create_string_scanner('-')
+    assert_nil(s.scan_integer)
+    assert_equal(0, s.pos)
+    refute_predicate(s, :matched?)
+
+    s = create_string_scanner('+')
+    assert_nil(s.scan_integer)
+    assert_equal(0, s.pos)
+    refute_predicate(s, :matched?)
+
     huge_integer = '1' * 2_000
     s = create_string_scanner(huge_integer)
     assert_equal(huge_integer.to_i, s.scan_integer)