RFR: 8252997: Null-proofing for linker_md.c [v2]

[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

Adam Farley has refreshed the contents of this pull request, and previous commits have been removed. The incremental
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  8252997: Null-proofing for linker_md.c

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/246/files
  - new: https://git.openjdk.java.net/jdk/pull/246/files/89e43419..cfaf382c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=246&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=246&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 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