Use null error_msg for pic images

Reduces 20ms of overhead for reading proc maps when the map can't be
placed at the optimal address.

Bug: 34927277

Test: test-art-host && device boot

Change-Id: Ib91fff2b832f61c49e67edcd2b7a222ab4409984
diff --git a/runtime/mem_map.cc b/runtime/mem_map.cc
index dce56b3..93c212b 100644
--- a/runtime/mem_map.cc
+++ b/runtime/mem_map.cc
@@ -400,7 +400,7 @@
     // reuse means it is okay that it overlaps an existing page mapping.
     // Only use this if you actually made the page reservation yourself.
     CHECK(expected_ptr != nullptr);
-
+    DCHECK(error_msg != nullptr);
     DCHECK(ContainedWithinExistingMap(expected_ptr, byte_count, error_msg))
         << ((error_msg != nullptr) ? *error_msg : std::string());
     flags |= MAP_FIXED;