leapdb: fix ordered comparison against NULL pointer

leapdb: fix ordered comparison against NULL pointer

fgets returns either a valid pointer with the same value as its first
argument or NULL on error or EOF.

GCC 12.2.0 -Wextra warns against relational comparison of the return
value:

  leapdb.c:127:38: warning: ordered comparison of pointer with integer zero [-Wextra]

For clarity, and because the C standard doesn't mandate that valid pointers
have to compare greater than the null pointer constant, replace the
relational expression with an equality expression