Use memcpy instead of strncpy

It's not supposed to have a '\0' at the end and if it's 8 letters it
should end in a letter, so str* functions aren't the right choice.
Silences a compiler warning.

src/profiler/heapsnapshot.c

@@ -902,8 +902,8 @@ void serialize_attribute_stream(MVMThreadContext *tc, MVMHeapSnapshotCollection
     }
 
     {
-        char namebuf[8];
-        strncpy(namebuf, name, 8);
+        char namebuf[8] = {0};
+        memcpy(namebuf, name, 8);
         fwrite(namebuf, 8, 1, fh);
     }