RFR: 8252997: Null-proofing for linker_md.c

[Adam Farley]

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.

Bug link: https://bugs.openjdk.java.net/browse/JDK-8252997

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Commit messages:
 - 8252997: Null-proofing for linker_md.c

Changes: https://git.openjdk.java.net/jdk/pull/246/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=246&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252997
  Stats: 6 lines in 2 files changed: 4 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/246.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/246/head:pull/246

PR: https://git.openjdk.java.net/jdk/pull/246