logging: Fix Not enough tailroom msgb_put in _output_buf callers

The function clearly specified in its documentation that the number of
bytes written to the out buffer were being returned. However, the value
returned was "the number of characters (excluding the terminating null
byte) which would have been written to the final string if enough space
had been available.", aka snprintf-style.
The 2 callers of that function were not expecting it, so if a long
enough buffer was passed, the program asserted.

Closes: OS#5383
Change-Id: I8d71bd1a0dad37606acb8302b05c2ae338112e57

src/logging.c

@@ -601,7 +601,8 @@ static int _output_buf(char *buf, int buf_len, struct log_target *target, unsign
 		OSMO_SNPRINTF_RET(ret, rem, offset, len);
 	}
 err:
-	buf[buf_len-1] = '\0';
+	len = OSMO_MIN(buf_len - 1, len);
+	buf[len] = '\0';
 	return len;
 }