basic: nicer xsprintf and xstrftime assert messages

It's nicer if the assertion failure message from a bad use of xsprintf
actually mentions xsprintf instead of the expression the macro is
implemented as.

The assert_message_se macro was added in the previous commit as an
internal helper, but it can also be used for customizing assertion
failure messages like in this case.

Example:
  char buf[10];
  xsprintf(buf, "This is a %s message.\n", "long");

Before:
  Assertion '(size_t) snprintf(buf, ELEMENTSOF(buf), "This is a %s
  message.\n", "long") < ELEMENTSOF(buf)' failed at foo.c:6, function
  main(). Aborting.

After:
  Assertion 'xsprintf: buf[] must be big enough' failed at foo.c:6,
  function main(). Aborting.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/basic/util.h b/src/basic/util.h
index c7dff9a86d..8abaa740b2 100644
--- a/src/basic/util.h
+++ b/src/basic/util.h
@@ -374,7 +374,9 @@ DEFINE_TRIVIAL_CLEANUP_FUNC(cpu_set_t*, CPU_FREE);
 
 cpu_set_t* cpu_set_malloc(unsigned *ncpus);
 
-#define xsprintf(buf, fmt, ...) assert_se((size_t) snprintf(buf, ELEMENTSOF(buf), fmt, __VA_ARGS__) < ELEMENTSOF(buf))
+#define xsprintf(buf, fmt, ...) \
+        assert_message_se((size_t) snprintf(buf, ELEMENTSOF(buf), fmt, __VA_ARGS__) < ELEMENTSOF(buf), \
+                          "xsprintf: " #buf "[] must be big enough")
 
 int files_same(const char *filea, const char *fileb);