[tor-dev] small patch to allow gcc -fno-common for better ASAN coverage

Per https://code.google.com/p/address-sanitizer/wiki/Flags one benefits from compiling with -fno-common when -fsanitize=address is active The attached patch converts the single common variable to a global/extern variable. Seems a good idea to me. Have run this with no issues, but more ASAN coverage is better than less.