ICU-20824 Fix - 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;
 

icu4c/source/extra/scrptrun/srtest.cpp

@@ -16,6 +16,7 @@
 #include <stdio.h>
 
 char16_t testChars[] = {
+            '(', ')', '(', ')', '(', ')', 'x', '(', ')',
             0x0020, 0x0946, 0x0939, 0x093F, 0x0928, 0x094D, 0x0926, 0x0940, 0x0020,
             0x0627, 0x0644, 0x0639, 0x0631, 0x0628, 0x064A, 0x0629, 0x0020,
             0x0420, 0x0443, 0x0441, 0x0441, 0x043A, 0x0438, 0x0439, 0x0020,