abis: Check for failure of ftell

In case ftell -1 will be returned. Coverity complained that the
pos we pass to fseek might be negative. In case the ftell fails
we are at the last line for sure.

Fixes: Coverity CID 1040721
1 file changed
tree: fd870d3a78f39c033fb07e1774be5bf07dd120ca
  1. debian/
  2. hlrsync/
  3. linux-kernel/
  4. openbsc/
  5. wireshark/
  6. README