BUG/MINOR: sample: fix NULL dereference in the ltime/utime converters

BUG/MINOR: sample: fix NULL dereference in the ltime/utime converters

conv_time_common() returns NULL when alloc_trash_chunk() fails, and when
the format string expanded for "%N" does not fit in the trash chunk. Its
four callers, sample_conv_us_ltime(), sample_conv_ms_ltime(),
sample_conv_us_utime() and sample_conv_ms_utime(), dereference the
result without checking it.

A configuration using one of these converters with a long format, e.g.
ms_ltime("%N%N%N...") expanding to more than bufsize, is accepted by
"haproxy -c", and the first request evaluating it crashes the process
with a NULL dereference. A failed allocation at runtime has the same
effect with an ordinary format.

Let's make the converter fail instead, like on its other error paths.

This fixes GitHub issue #3545.

This must be backported to all stable versions up to 3.0, as commit
5e63e1636 introduced the first version of conv_time_common() with a
possible NULL return.