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

openbsc/src/libbsc/abis_nm.c

@@ -829,6 +829,11 @@ static int is_last_line(FILE *stream)
 
 	/* check if we're sending the last line */
 	pos = ftell(stream);
+
+	/* Did ftell fail? Then we are at the end for sure */
+	if (pos < 0)
+		return 1;
+
 	if (!fgets(next_seg_buf, sizeof(next_seg_buf)-2, stream)) {
 		fseek(stream, pos, SEEK_SET);
 		return 1;