Fix integer issues leading to out of bounds access in fx_ge_text.cpp.

- Using |-skew| to get positive index, which doesn't work when skew is
  INT_MIN
- Incorrect logic when determining when to use |-skew| as an index.

R=tsepez@chromium.org,weili@chromium.org
BUG=chromium:601362

Review URL: https://codereview.chromium.org/1875673004
diff --git a/BUILD.gn b/BUILD.gn
index 380eee9..eecdb5a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1687,6 +1687,7 @@
     "core/fpdfapi/fpdf_parser/fpdf_parser_decode_embeddertest.cpp",
     "core/fpdfapi/fpdf_render/fpdf_render_loadimage_embeddertest.cpp",
     "core/fpdfapi/fpdf_render/fpdf_render_pattern_embeddertest.cpp",
+    "core/fxge/ge/fx_ge_text_embeddertest.cpp",
     "fpdfsdk/fpdf_dataavail_embeddertest.cpp",
     "fpdfsdk/fpdfdoc_embeddertest.cpp",
     "fpdfsdk/fpdfedit_embeddertest.cpp",