avoid potential out-of-bounds array access (undefined behaviour)

This is a backport of a fix from mapnik clone of this class:
mapnik/mapnik#4096

Also suggested to icu as:
https://unicode-org.atlassian.net/browse/ICU-20824

We encountered it in mapnik and fixed it in our own mapnik 3.0 fork. I think it worth backporting the fix to here as well.

Author: oshai

icu4c/source/extra/scrptrun/scrptrun.cpp

@@ -155,6 +155,7 @@ UBool ScriptRun::next()
             if ((pairIndex & 1) == 0) {
                 parenStack[++parenSP].pairIndex = pairIndex;
                 parenStack[parenSP].scriptCode  = scriptCode;
+                startSP = parenSP;
             } else if (parenSP >= 0) {
                 int32_t pi = pairIndex & ~1;