Add defined in if().

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@58562 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/scan-build b/utils/scan-build
index 4ad8a74..41dd29e 100755
--- a/utils/scan-build
+++ b/utils/scan-build
@@ -1145,7 +1145,7 @@
 
 $ENV{'CCC_ANALYZER_ANALYSIS'} = join ' ',@AnalysesToRun;
 
-if ($StoreModel) {
+if (defined $StoreModel) {
   $ENV{'CCC_ANALYZER_STORE_MODEL'} = $StoreModel;
 }