Fix segmentations failure in error.c gumbo_caret_diagnostic_to_string

Makefile.am

@@ -93,7 +93,8 @@ gumbo_test_SOURCES = \
 				tests/tokenizer.cc \
 				tests/test_utils.cc \
 				tests/utf8.cc \
-				tests/vector.cc
+				tests/vector.cc \
+				tests/error.cc
 gumbo_test_DEPENDENCIES = libgumbo.la
 gumbo_test_LDADD = libgumbo.la
 

src/error.c

@@ -140,7 +140,7 @@ static const char* find_last_newline(
     // There may be an error at EOF, which would be a nul byte.
     assert(*c || c == error_location);
   }
-  return c == original_text ? c : c + 1;
+  return c == original_text || c == error_location ? c : c + 1;
 }
 
 // Finds the next newline in the original source buffer from a given byte

tests/error.cc

@@ -0,0 +1,41 @@
+#include "gumbo.h"
+#include "parser.h"
+#include "error.h"
+
+#include <string>
+
+#include "gtest/gtest.h"
+#include "test_utils.h"
+
+namespace {
+
+class GumboErrorTest : public ::testing::Test {
+ protected:
+  GumboErrorTest() {}
+
+  virtual ~GumboErrorTest() {
+
+  }
+};
+
+TEST_F(GumboErrorTest, NewlineAfterLessThanSymbol) {
+  const GumboOptions *options = &kGumboDefaultOptions;
+  const char *input = "<\n";
+  size_t input_len = strlen(input);
+  GumboOutput *output = gumbo_parse_with_options(options, input, input_len);
+  GumboVector *errors = &output->errors;
+  GumboParser parser = { ._options = options };
+  GumboStringBuffer msg;
+
+  gumbo_string_buffer_init(&parser, &msg);
+  for (size_t i=0; i < errors->length; i++) {
+    GumboError *err = (GumboError *)errors->data[i];
+    gumbo_string_buffer_clear(&parser, &msg);
+    gumbo_caret_diagnostic_to_string(&parser, err, input, &msg);
+  }
+  gumbo_string_buffer_destroy(&parser, &msg);
+
+  gumbo_destroy_output(options, output);
+}
+
+}