Set return value to FALSE (0) in estimate_dt in libxdrf.c

Set return value to FALSE (0) in estimate_dt in libxdrf.c

To be on the safe side the function to estimate the time
step betwen frames was fixed such that the return value
bOK is always set even if the file was missing or empty.
In addition the function is made static since it is not
used outside the file.

Change-Id: I8706a23b7a198f9d0718f1bfcb4a7770b2e6e53c