nc_inq_format segfaults when passed a NULL for formatp

[edhartnett]

I found this while increasing test coverage in ncfunc.c under #677. When I try this code:

if (nc_inq_format(ncid, NULL) != NC_EBADID) ERR;

I get:

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7b312a8 in NC3_inq_format (ncid=65578, formatp=0x0) at nc3internal.c:1599
1599		    *formatp = NC_FORMAT_CLASSIC;

I will fix.