Fix crash when scanning invalid regexp

GitHub: #52

As re2 will report the number of capturing groups for an invalid regular
expression as -1, we need to check whether a pattern is OK or not before
using the number to initialize any vectors, etc.

This was causing a crash when using RE2::Scanner#scan with an invalid
regular expression as we would attempt to initialize vectors with a
length of -1. Instead, set the number of capturing groups to 0 and rely
on re2's FindAndConsumeN returning false so we return nil to the user to
indicate no match was made.

ext/re2/re2.cc

@@ -1251,7 +1251,13 @@ static VALUE re2_regexp_scan(VALUE self, VALUE text) {
   c->input = new(nothrow) re2::StringPiece(StringValuePtr(text));
   c->regexp = self;
   c->text = text;
-  c->number_of_capturing_groups = p->pattern->NumberOfCapturingGroups();
+
+  if (p->pattern->ok()) {
+    c->number_of_capturing_groups = p->pattern->NumberOfCapturingGroups();
+  } else {
+    c->number_of_capturing_groups = 0;
+  }
+
   c->eof = false;
 
   return scanner;

spec/re2/scanner_spec.rb

@@ -45,6 +45,12 @@
       expect(scanner.scan).to be_nil
     end
 
+    it "returns nil if the regexp is invalid" do
+      r = RE2::Regexp.new('???', :log_errors => false)
+      scanner = r.scan("Foo bar")
+      expect(scanner.scan).to be_nil
+    end
+
     it "returns an empty array if the input is empty" do
       r = RE2::Regexp.new("")
       scanner = r.scan("")