Fix null terminator in Demangle function.

If the demangled name is longer than out_size, the null terminator is
missing from the output. This will cause a crash in the DemangleInplace()
function (symbolize.cc) when calling strlen on the buffer.

src/demangle.cc

@@ -1351,6 +1351,9 @@ bool Demangle(const char* mangled, char* out, size_t out_size) {
   }
 
   std::copy_n(unmangled.get(), std::min(n, out_size), out);
+  if(n > out_size) {
+    out[out_size-1] = '\0';
+  }
   return status == 0;
 #else
   State state;