Fix search for colon in IsIsoTimeFormat()

Look for colon separator relative to index, not at fixed offset 2.
This has been wrong since at least 2018. Unfortunately, testing is
thin because IsoTime2Num test is disabled.

Change-Id: I7c06f900a9892ca9be10bcf3b674be33c8ee3d0e
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/109030
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

fxjs/xfa/cfxjse_formcalc_context.cpp

diff --git a/fxjs/xfa/cfxjse_formcalc_context.cpp b/fxjs/xfa/cfxjse_formcalc_context.cpp
index 790ec94..695cc25 100644
--- a/fxjs/xfa/cfxjse_formcalc_context.cpp
+++ b/fxjs/xfa/cfxjse_formcalc_context.cpp
@@ -630,7 +630,7 @@
     if (FXSYS_atoi(szBuffer) > 60) {
       return false;
     }
-    if (pData[2] == ':') {
+    if (iIndex + 2 < iZone && pData[iIndex + 2] == ':') {
       if (iPos == 0) {
         iHour = FXSYS_atoi(szBuffer);
         ++iPos;
@@ -701,7 +701,7 @@
     if (FXSYS_atoi(szBuffer) > 60) {
       return false;
     }
-    if (pData[2] == ':') {
+    if (iIndex + 2 < pData.size() && pData[iIndex + 2] == ':') {
       if (iPos == 0) {
         iZoneHour = FXSYS_atoi(szBuffer);
       } else if (iPos == 1) {