Fix: clear_string_field(): set first character to 0

Fix: clear_string_field(): set first character to 0

The API documentation of bt_field_string_clear() says:

> Clears the string field `field`, making its value an empty string.

clear_string_field(), called by bt_field_string_clear(), sets the string
field's buffer's length to 0 without setting its first character to 0 to
make the string field empty from bt_field_string_get_value()'s point of
view.

This patch makes clear_string_field() also set the string field's
buffer's first character to 0.

Reported by Brice Videau (@Kerilk) in
<https://github.com/efficios/babeltrace/pull/116>.

Reported-by: Brice Videau <bvideau@anl.gov>
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I964599ecfbea33a7cb583dcf7cf25e6cffbc30b6
Reviewed-on: https://review.lttng.org/c/babeltrace/+/4528
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>