media: pci: ivtv: Add check for DMA map result

In the Linux kernel, the following vulnerability has been resolved: media: pci: ivtv: Add check for DMA map result In case DMA fails, 'dma->SG_length' is 0. This value is later used to access 'dma->SGarray[dma->SG_length - 1]', which will cause out of bounds access. Add check to return early on invalid value. Adjust warnings accordingly. Found by Linux Verification Center (linuxtesting.org) with SVACE.