Fix some memory leaks, potential errors.

Fix a couple of memory leaks in tests.

Fix a potential case that length could be zero.

Bug: 23762183

Test: Ran unit tests, then ran valgrind on unit tests and verified no leaks.
Change-Id: I5f7602aa0204db0488a8e305250658cdb6e977d0
diff --git a/libunwindstack/tests/MapInfoTest.cpp b/libunwindstack/tests/MapInfoTest.cpp
index c846ad7..6e47dc0 100644
--- a/libunwindstack/tests/MapInfoTest.cpp
+++ b/libunwindstack/tests/MapInfoTest.cpp
@@ -75,7 +75,7 @@
   // Make sure this test is valid.
   info.end = 0x101;
   memory.reset(info.CreateMemory(getpid()));
-  ASSERT_FALSE(info.CreateMemory(getpid()) == nullptr);
+  ASSERT_TRUE(memory.get() != nullptr);
 }
 
 // Verify that if the offset is non-zero but there is no elf at the offset,
@@ -212,8 +212,8 @@
   MapInfo info{.start = start, .end = start + 1024, .offset = 0, .name = ""};
 
   // The map contains garbage, but this should still produce an elf object.
-  Elf* elf = info.GetElf(getpid(), false);
-  ASSERT_TRUE(elf != nullptr);
+  std::unique_ptr<Elf> elf(info.GetElf(getpid(), false));
+  ASSERT_TRUE(elf.get() != nullptr);
   ASSERT_FALSE(elf->valid());
 
   ASSERT_EQ(0, munmap(map, 1024));