RFR: 8281771: Crash in java_lang_invoke_MethodType::print_signature

[Leonid Mesnik]

The crash happens in java.lang.invoke.MethodType.<init> when VM tries to print this objec while rtype and ptypes are not initialized yet. So the fix is just to check if they are not NULL.

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Commit messages:
 - 8281771: Crash in java_lang_invoke_MethodType::print_signature

Changes: https://git.openjdk.java.net/jdk/pull/7470/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7470&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281771
  Stats: 13 lines in 1 file changed: 9 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7470.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7470/head:pull/7470

PR: https://git.openjdk.java.net/jdk/pull/7470