libdl: Fix allocator NULL check (Coverity 1255328) refs #2191

The address returned from the allocator call was not checked when zeroing
the memory.

cpukit/libdl/rtl-allocator.c

@@ -61,7 +61,7 @@ rtems_rtl_alloc_new (rtems_rtl_alloc_tag_t tag, size_t size, bool zero)
     printf ("rtl: alloc: new: %s addr=%p size=%zu\n",
             rtems_rtl_trace_tag_label (tag), address, size);
 
-  if (zero)
+  if (address && zero)
     memset (address, 0, size);
 
   return address;