ICU-23112 Add check for avoid underflow in MeasureUnit::getIdentifier

Author: wooffie

icu4c/source/i18n/measunit.cpp

@@ -2650,6 +2650,9 @@ const char *MeasureUnit::getSubtype() const {
 }
 
 const char *MeasureUnit::getIdentifier() const {
+    if (fTypeId < 0 || fSubTypeId < 0) {
+        return "";
+    }
     return fImpl ? fImpl->identifier.data() : gSubTypes[getOffset()];
 }