astfd.c: Avoid calling fclose with NULL argument.

astfd.c: Avoid calling fclose with NULL argument.

Don't pass through a NULL argument to fclose, which is undefined
behavior, and instead return -1 and set errno appropriately. This
also avoids a compiler warning with glibc 2.38 and newer, as glibc
commit 71d9e0fe766a3c22a730995b9d024960970670af
added the nonnull attribute to this argument.

Resolves: #900