aft.c: Silence scan-build warning.

aft.c: Silence scan-build warning.

scan-build issues the following warning in prepare_ls_row():

	aft.c:1338:3: warning: Assigned value is garbage or undefined
			GET_NUM_DIGITS(score, &num_digits);
			^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	aft.c:719:21: note: expanded from:
		typeof((x)) _tmp = PARA_ABS(x); \
				   ^
	./para.h:50:2: note: expanded from:
		typeof(x) _x = (x); \
		^              ~~~

This is because clang can not prove that the "score" variable is
always initialized at this point. However, I can: The problematic
GET_NUM_DIGITS() statement is only executed if LS_FLAG_ADMISSIBLE_ONLY
is set in options->flags, but in this case "score" will be set by
get_score_and_aft_row() because this function only fails to do so if it
returns negative. In this case we return early from prepare_ls_row().

Let's make it easier for clang and initialize it anyway.