https://github.com/unicode-org/icu/commit/b33869b304cff387d544eaa5e820b93a39e737bf

--- icu/source/common/ucasemap.cpp.orig
+++ icu/source/common/ucasemap.cpp
@@ -104,11 +104,14 @@
 
     UErrorCode bufferStatus = U_ZERO_ERROR;
     int32_t length=uloc_getName(locale, csm->locale, (int32_t)sizeof(csm->locale), &bufferStatus);
-    if(bufferStatus==U_BUFFER_OVERFLOW_ERROR || length==sizeof(csm->locale)) {
+    if(bufferStatus==U_BUFFER_OVERFLOW_ERROR || (U_SUCCESS(bufferStatus) && length==sizeof(csm->locale))) {
+        bufferStatus = U_ZERO_ERROR;
         /* we only really need the language code for case mappings */
-        length=uloc_getLanguage(locale, csm->locale, (int32_t)sizeof(csm->locale), pErrorCode);
+        length=uloc_getLanguage(locale, csm->locale, (int32_t)sizeof(csm->locale), &bufferStatus);
     }
-    if(length==sizeof(csm->locale)) {
+    if(U_FAILURE(bufferStatus)) {
+        *pErrorCode=bufferStatus;
+    } else if(length==sizeof(csm->locale)) {
         *pErrorCode=U_BUFFER_OVERFLOW_ERROR;
     }
     if(U_SUCCESS(*pErrorCode)) {