[Fix] To confirm curl exists before detecting curl libz feature

nvm.sh

@@ -82,10 +82,10 @@ nvm_curl_libz_support() {
 nvm_get_latest() {
   local NVM_LATEST_URL
   local CURL_COMPRESSED_FLAG
-  if nvm_curl_libz_support; then
-    CURL_COMPRESSED_FLAG="--compressed"
-  fi
   if nvm_has "curl"; then
+    if nvm_curl_libz_support; then
+      CURL_COMPRESSED_FLAG="--compressed"
+    fi
     NVM_LATEST_URL="$(curl "${CURL_COMPRESSED_FLAG:-}" -q -w "%{url_effective}\n" -L -s -S http://latest.nvm.sh -o /dev/null)"
   elif nvm_has "wget"; then
     NVM_LATEST_URL="$(wget http://latest.nvm.sh --server-response -O /dev/null 2>&1 | command awk '/^  Location: /{DEST=$2} END{ print DEST }')"