Fix -Wnullable-to-nonnull-conversion error

Fix the build errors like https://android-build.googleplex.com/builds/pending/P56005836/aosp_arm64-userdebug/latest/view/logs/build.log

Bugs: b/245972273
Test: mm -j
Change-Id: I8edeace5afddc2cf6783899c889a3f2a542156a3
diff --git a/libs/binder/MemoryHeapBase.cpp b/libs/binder/MemoryHeapBase.cpp
index 8fe1d2b..3da06ba 100644
--- a/libs/binder/MemoryHeapBase.cpp
+++ b/libs/binder/MemoryHeapBase.cpp
@@ -78,7 +78,7 @@
         if (SEAL_FLAGS && (fcntl(fd, F_ADD_SEALS, SEAL_FLAGS) == -1)) {
             ALOGE("MemoryHeapBase: MemFD %s sealing with flags %x failed with error  %s", name,
                   SEAL_FLAGS, strerror(errno));
-            munmap(mBase, mSize);
+            if (mNeedUnmap) munmap(mBase, mSize);
             mBase = nullptr;
             mSize = 0;
             close(fd);