basic/stdio-util: use a fixed message in xsprintf

We put the name of the variable in the message, but it is a local variable
and the name does not have global meaning. We end up with pointless copies
of the error string:

$ strings build/libsystemd.so.0.40.0 | grep 'big enough'
xsprintf: p[] must be big enough
xsprintf: error[] must be big enough
xsprintf: prefix[] must be big enough
xsprintf: pty[] must be big enough
xsprintf: mode[] must be big enough
xsprintf: t[] must be big enough
xsprintf: s[] must be big enough
xsprintf: spid[] must be big enough
xsprintf: header_priority[] must be big enough
xsprintf: header_pid[] must be big enough
xsprintf: path[] must be big enough
xsprintf: buf[] must be big enough

The error message already shows the file, line, and function name, which
is enough to identify the problem:

  Assertion 'xsprintf: buffer too small' failed at src/test/test-string-util.c:20, function test_xsprintf(). Aborting.

src/basic/stdio-util.h

@@ -19,7 +19,7 @@ static inline char* snprintf_ok(char *buf, size_t len, const char *format, ...)
 }
 
 #define xsprintf(buf, fmt, ...) \
-        assert_message_se(snprintf_ok(buf, ELEMENTSOF(buf), fmt, ##__VA_ARGS__), "xsprintf: " #buf "[] must be big enough")
+        assert_message_se(snprintf_ok(buf, ELEMENTSOF(buf), fmt, ##__VA_ARGS__), "xsprintf: buffer too small")
 
 #define VA_FORMAT_ADVANCE(format, ap)                                   \
 do {                                                                    \