Null-proofing for linker_md.c

In the platform-specific implementations of linker_md.c, we see the dll_build_name methods begin with a call to jvmtiAllocate.

We then appear to rush ahead and try to use that variable without checking for a null.

I propose moving the null check to the point *before* we try to use that variable, to avoid any possible SEGV.

https://github.com/openjdk/jdk/pull/246