Fix compilation failures of lalapps HeterodyneSearch codes under gcc7.X
This fixes compilation failures (due to string truncation warnings) of
pulsar_parameter_estimation.c under gcc7.2.1 (on Ubuntu Buster). To do this it checks the return value of
snprint as in e4e09e9c. This has been tested and builds using the Docker image from here with the buster tag. This is related to #1 (closed).