Set variable in Windows scripts to default value

This commit sets the bad_env_var to a default value of 0 so that in case
no bad environment variables are encountered the variable is still
defined and does not later cause a parse error in the execution of these
batch files.

Relates #17675

distribution/src/main/resources/bin/elasticsearch.bat

@@ -6,6 +6,7 @@ TITLE Elasticsearch ${project.version}
 SET params='%*'
 
 rem TODO: remove for Elasticsearch 6.x
+set bad_env_var=0
 if not "%ES_MIN_MEM%" == "" set bad_env_var=1
 if not "%ES_MAX_MEM%" == "" set bad_env_var=1
 if not "%ES_HEAP_SIZE%" == "" set bad_env_var=1

distribution/src/main/resources/bin/service.bat

@@ -4,6 +4,7 @@ SETLOCAL enabledelayedexpansion
 TITLE Elasticsearch Service ${project.version}
 
 rem TODO: remove for Elasticsearch 6.x
+set bad_env_var=0
 if not "%ES_MIN_MEM%" == "" set bad_env_var=1
 if not "%ES_MAX_MEM%" == "" set bad_env_var=1
 if not "%ES_HEAP_SIZE%" == "" set bad_env_var=1