Avoid compilation warnings in optimized builds

Avoid compilation warnings in optimized builds

* src/process.c (Fmake_network_process):
* src/image.c (xbm_scan, gif_load):
* src/fileio.c (Frename_file):
* src/data.c (Fmake_local_variable):
* src/buffer.c (fix_start_end_in_overlays):
* lib-src/etags.c (process_file_name): Mark variables with UNINIT
to avoid compiler warnings in optimized builds.