Fix crash in hardcoded_nonce analyzer

Change-Id: If5ed3709d6e1ddced1be555477dd0f5451aab901
Signed-off-by: Cosmin Cojocar <ccojocar@google.com>

analyzers/hardcoded_nonce.go

@@ -197,12 +197,20 @@ func isFuncContainsCryptoRand(funcCall *ssa.Function) (bool, error) {
 }
 
 func addToVarsMap(value ssa.Value, mapToAddTo map[string]*ssa.Value) {
-	key := value.Name() + value.Type().String() + value.String() + value.Parent().String()
+	var parent string
+	if value.Parent() != nil {
+		parent = value.Parent().String()
+	}
+	key := value.Name() + value.Type().String() + value.String() + parent
 	mapToAddTo[key] = &value
 }
 
 func isContainedInMap(value ssa.Value, mapToCheck map[string]*ssa.Value) bool {
-	key := value.Name() + value.Type().String() + value.String() + value.Parent().String()
+	var parent string
+	if value.Parent() != nil {
+		parent = value.Parent().String()
+	}
+	key := value.Name() + value.Type().String() + value.String() + parent
 	_, contained := mapToCheck[key]
 	return contained
 }