null check fTeam in CoreDumper::Dump()

gcc warns "writing 4 bytes into a region of size 0"
on ARM64 (-march=armv8.1-a -mno-outline-atomics)
if it thinks fTeam can be null in atomic_or.

Change-Id: I44c88dadf0cee6e3e54d2baaf0453ae70aa1f3c1
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6838
Reviewed-by: Niels Sascha Reedijk <[email protected]>
Tested-by: Commit checker robot <[email protected]>

src/system/kernel/debug/core_dump.cpp

@@ -777,6 +777,11 @@ struct CoreDumper {
 
 	status_t Dump(const char* path, bool killTeam)
 	{
+		// gcc thinks fTeam may be null in atomic_or
+        // and warn which causes error on some configs
+		if (fTeam == NULL)
+			return B_ERROR;
+
 		// the path must be absolute
 		if (path[0] != '/')
 			return B_BAD_VALUE;