image-sgi: Return early if sgiOpenFile fails

image-sgi: Return early if sgiOpenFile fails

sgiOpenFile can fail when passing an image that both has large
dimensions and which also uses RLE compression.  In that case,
sgiOpenFile attempts to allocate space for compression-related tables
and these allocations may fail due to the requested size, causing
sgiOpenFile to return NULL.  Without this fix, the NULL pointer gets
dereferenced, which crashes the filter process.

Compare https://en.wikipedia.org/wiki/Silicon_Graphics_Image#Header
for an overview of the SGI image headere.