ART: Fix generic_ptr handling in GetMethodName

Do not write null into a null pointer.

Bug: 31684812
Test: m test-art-host-run-test-910-methods
Change-Id: I79a255c1403ca9621390c9cdc09d2381c26fffb6
diff --git a/runtime/openjdkjvmti/ti_method.cc b/runtime/openjdkjvmti/ti_method.cc
index e391a9d..ffa5ac7 100644
--- a/runtime/openjdkjvmti/ti_method.cc
+++ b/runtime/openjdkjvmti/ti_method.cc
@@ -78,7 +78,9 @@
   }
 
   // TODO: Support generic signature.
-  *generic_ptr = nullptr;
+  if (generic_ptr != nullptr) {
+    *generic_ptr = nullptr;
+  }
 
   // Everything is fine, release the buffers.
   name_copy.release();