mediadrm_fuzzer set up AString poorly

Code set up a null pointer and passed it to routines that expected it to
be non-null.

Bug: 221403640
Test: build
Change-Id: Iaaac4d3b59f9663dacbb6a080b9f97689305bb7a
diff --git a/drm/libmediadrm/CryptoHalAidl.cpp b/drm/libmediadrm/CryptoHalAidl.cpp
index 3dc62e9..bda664a 100644
--- a/drm/libmediadrm/CryptoHalAidl.cpp
+++ b/drm/libmediadrm/CryptoHalAidl.cpp
@@ -353,7 +353,9 @@
 
     err = statusAidlToStatusT(statusAidl);
     std::string msgStr(statusAidl.getMessage());
-    *errorDetailMsg = toString8(msgStr);
+    if (errorDetailMsg != nullptr) {
+        *errorDetailMsg = toString8(msgStr);
+    }
     if (err != OK) {
         ALOGE("Failed on decrypt, error description:%s", statusAidl.getDescription().c_str());
         return err;
@@ -415,4 +417,4 @@
 
     return DrmUtils::GetLogMessagesAidl<ICryptoPluginAidl>(mPlugin, logs);
 }
-}  // namespace android
\ No newline at end of file
+}  // namespace android