logging: fix NULL pointer dereference in _output_buf()

In the _output_buf() we explicitly initialize only the 'buf' and 'len'
fields of the struct osmo_strbuf, leaving the 'pos' field implicitly
initialized to NULL.  Later, in this function, 'sb.pos' is passed to
ctime_r() and strlen(), leading to a NULL pointer dereference (segfault)
in certain scenarios.

This situation can occur when color logging is disabled or when
a specific logging subsystem has no associated color.  Any application
using libosmocore's logging API would crash with the following config:

log stderr
 logging filter all 1
 logging timestamp 1
 logging color 0

Fix this by initializing the 'pos' field explicitly.

Change-Id: I7ec9badf525e03e54e10b725d820c636eaa3fd1c
Fixes: d71331bc "logging: fix nul octets in log output / use osmo_strbuf"
Fixes: CID#336550

src/core/logging.c

@@ -488,7 +488,7 @@ static int _output_buf(char *buf, int buf_len, struct log_target *target, unsign
 {
 	int ret;
 	const char *c_subsys = NULL;
-	struct osmo_strbuf sb = { .buf = buf, .len = buf_len };
+	struct osmo_strbuf sb = { .buf = buf, .pos = buf, .len = buf_len };
 
 	/* safety net in case of encountering errors and returning nothing */
 	buf[0] = '\0';