Revert "Use memcpy instead of strncpy"

This reverts commit 9294cbf.

memcpy can lead to a buffer overflow if name is shorter than 8 bytes. The
compiler warning aside, strncpy seems like exactly the right tool for the job
as we want at most 8 bytes and don't care for any trailing \0, but are OK with
zero padding at the end.

src/profiler/heapsnapshot.c

@@ -890,8 +890,8 @@ void serialize_attribute_stream(MVMThreadContext *tc, MVMHeapSnapshotCollection
     }
 
     {
-        char namebuf[8] = {0};
-        memcpy(namebuf, name, 8);
+        char namebuf[8];
+        strncpy(namebuf, name, 8);
         fwrite(namebuf, 8, 1, fh);
     }