Fix nullptr dereference.

A previous cl missed adding a return in the new path which could result
in a crash if the map info is null. Add the return back.

Added a new test to catch this case.

Bug: 166188440

Test: Unit tests pass. Verified the new test crashes on the old code.
Change-Id: I9420b47dae0f880493c0e6a60d97bb3468ff2906
diff --git a/libunwindstack/Unwinder.cpp b/libunwindstack/Unwinder.cpp
index b904632..4e7096e 100644
--- a/libunwindstack/Unwinder.cpp
+++ b/libunwindstack/Unwinder.cpp
@@ -76,6 +76,7 @@
   } else {
     frame->rel_pc = dex_pc;
     warnings_ |= WARNING_DEX_PC_NOT_IN_MAP;
+    return;
   }
 
   if (!resolve_names_) {