Fix null-termination bug in Stacktrace::ToCString.

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 19252)
+++ runtime/vm/object.cc	(working copy)
@@ -13036,7 +13036,7 @@
     frame_strings.Add(chars);
   }
 
-  // Now concatentate the frame descriptions into a single C string.
+  // Now concatenate the frame descriptions into a single C string.
   chars = isolate->current_zone()->Alloc<char>(total_len + 1);
   intptr_t index = 0;
   for (intptr_t i = 0; i < frame_strings.length(); i++) {
@@ -13045,6 +13045,7 @@
                          "%s",
                          frame_strings[i]);
   }
+  chars[total_len] = '\0';
   return chars;
 }