Handle out of bound indices in PDFiumPage::GetTextRunInfo().

Check to see if the |char_index| is within bounds, and return early if
it is. Otherwise, the code internal to PDFiumPage, which only works with
valid indices, will behave badly.

Bug: 996629
Change-Id: I71c27b1039e6d997d3ce5613e8df7a30882e8347
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1767606
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Kevin Babbitt <kbabbitt@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#690041}

pdf/pdfium/pdfium_page.cc

@@ -255,12 +255,25 @@ void PDFiumPage::GetTextRunInfo(int start_char_index,
                                 uint32_t* out_len,
                                 double* out_font_size,
                                 pp::FloatRect* out_bounds) {
+  if (start_char_index < 0) {
+    *out_len = 0;
+    *out_font_size = 0;
+    *out_bounds = pp::FloatRect();
+    return;
+  }
+
   FPDF_PAGE page = GetPage();
   FPDF_TEXTPAGE text_page = GetTextPage();
   int chars_count = FPDFText_CountChars(text_page);
 
   int char_index = GetFirstNonUnicodeWhiteSpaceCharIndex(
       text_page, start_char_index, chars_count);
+  if (char_index >= chars_count) {
+    *out_len = 0;
+    *out_font_size = 0;
+    *out_bounds = pp::FloatRect();
+    return;
+  }
 
   pp::FloatRect start_char_rect =
       GetFloatCharRectInPixels(page, text_page, char_index);