include: cbprintf: Fix call to memcpy with null pointer

[tpambor]

cbprintf_package_convert may invoke z_cbprintf_cpy with a null pointer to buf and zero length to indicate a flush operation.

zephyr/lib/os/cbprintf_packaged.c

Lines 1166 to 1167 in f2c37c4

	/* Empty call (can be interpreted as flushing) */
	(void)cb(NULL, 0, ctx);

This triggers an error from the undefined behavior sanitizer due to memcpy being called with a null src:

zephyr/include/zephyr/sys/cbprintf.h:536:45: runtime error: null pointer passed as argument 2, which is declared to never be null
/usr/include/string.h:44:28: note: nonnull attribute specified here
    #0 0x8186fa9 in z_cbprintf_cpy zephyr/include/zephyr/sys/cbprintf.h:536:2
    #1 0x816bd79 in cbprintf_package_convert zephyr/lib/os/cbprintf_packaged.c:1167:8
    #2 0x81865c7 in cbprintf_package_copy zephyr/include/zephyr/sys/cbprintf.h:586:9
    #3 0x81865c7 in z_impl_z_log_msg_static_create zephyr/subsys/logging/log_msg.c:323:10
<snip>

SUMMARY: UndefinedBehaviorSanitizer: invalid-null-argument zephyr/include/zephyr/sys/cbprintf.h:536:45

Passing a null pointer as source to memcpy is undefined behavior according to the C standard.

The behavior is undefined if either dest or src is an invalid or null pointer.

This PR adds a fix by exiting early in z_cbprintf_cpy when length is zero.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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