ICU-20168 Fix memory leak in error path, add a free statement. (#175)

diff --git a/icu4c/source/common/uloc_tag.cpp b/icu4c/source/common/uloc_tag.cpp
index c718de7..17af4f7 100644
--- a/icu4c/source/common/uloc_tag.cpp
+++ b/icu4c/source/common/uloc_tag.cpp
@@ -1507,6 +1507,7 @@
             attrBufIdx += (len + 1);
         } else {
             *status = U_ILLEGAL_ARGUMENT_ERROR;
+            uprv_free(attr);
             goto cleanup;
         }