Be robust in stack trace if source file name is missing from dex file.

Change-Id: Iaf39454bc5b82a2785c236400b0523eba5ab1ae9
diff --git a/src/object.cc b/src/object.cc
index 614df3c..0b6ad51 100644
--- a/src/object.cc
+++ b/src/object.cc
@@ -1109,6 +1109,9 @@
 }
 
 String* String::AllocFromModifiedUtf8(const char* utf) {
+  if (utf == NULL) {
+    return NULL;
+  }
   size_t char_count = CountModifiedUtf8Chars(utf);
   return AllocFromModifiedUtf8(char_count, utf);
 }