Fix sourcemap handling.
Fix sourcemap handling.

The old code worked in simple cases by chance but made pretty mangled
sourcemaps if there was an input source map from other transforms.

sverrejoh/browserify-hmr,sverrejoh/browserify-hmr,AgentME/browserify-hmr,royriojas/browserify-hmr,AgentME/browserify-hmr,sverrejoh/browserify-hmr,royriojas/browserify-hmr,royriojas/browserify-hmr,AgentME/browserify-hmr