[lsan] Fix free(NULL) interception during initialization

Previously an attempt to free a null pointer during initialization would
fail on ENSURE_LSAN_INITED assertion (since a null pointer is not owned
by DlsymAlloc).

Author: tmiasko

compiler-rt/lib/lsan/lsan_interceptors.cpp

@@ -77,6 +77,8 @@ INTERCEPTOR(void*, malloc, uptr size) {
 }
 
 INTERCEPTOR(void, free, void *p) {
+  if (!p)
+    return;
   if (DlsymAlloc::PointerIsMine(p))
     return DlsymAlloc::Free(p);
   ENSURE_LSAN_INITED;