Merge pull request #384 from dbrotz/fix-local-label-segfault

Check if parent exists for local label reference

test/asm/local-ref-without-parent.asm

@@ -0,0 +1,3 @@
+SECTION "sec", ROM0
+
+dw .test

test/asm/local-ref-without-parent.out

@@ -0,0 +1,2 @@
+ERROR: local-ref-without-parent.asm(3):
+    Local label reference '.test' in main scope

test/asm/local-ref-without-parent.out.pipe

@@ -0,0 +1,2 @@
+ERROR: -(3):
+    Local label reference '.test' in main scope