Support complicated variable references

Bug: 226974242
Test: go test
Change-Id: Iaec16f5c498e7c75c9ee5d53d3499efadfba16bc
diff --git a/androidmk/parser/make_strings.go b/androidmk/parser/make_strings.go
index 416e430..8afbe7e 100644
--- a/androidmk/parser/make_strings.go
+++ b/androidmk/parser/make_strings.go
@@ -234,10 +234,10 @@
 		if n != 0 {
 			split := splitFunc(s, n)
 			if n != -1 {
-				if len(split) > n {
+				if len(split) > n || len(split) == 0 {
 					panic("oops!")
 				} else {
-					n -= len(split)
+					n -= len(split) - 1
 				}
 			}
 			curMs.appendString(split[0])