lexmark_low: Adjust calculation of bytes available in read buffer

GCC warns that an unsigned value is passed to abs(), which may not be
intended. To resolve this warning, adjust the code so abs() is not used
and the behavior is more explicit.

backend/lexmark_low.c

@@ -5252,12 +5252,12 @@ read_buffer_bytes_available (Read_Buffer * rb)
 
   if (rb->empty)
     return rb->size;
-  else if ((size_t) abs (rb->writeptr - rb->readptr) < rb->linesize)
-    return 0;			/* ptrs are less than one line apart */
   else if (rb->writeptr < rb->readptr)
-    return (rb->readptr - rb->writeptr - rb->linesize);
+    return (size_t)(rb->readptr - rb->writeptr) < rb->linesize ? 0 :
+           (size_t)(rb->readptr - rb->writeptr) - rb->linesize;
   else
-    return (rb->size + rb->readptr - rb->writeptr - rb->linesize);
+    return (size_t)(rb->writeptr - rb->readptr) < rb->linesize ? 0 :
+           rb->size - (size_t)(rb->writeptr - rb->readptr) - rb->linesize;
 }
 
 SANE_Status