RFR: 8285279: ArgumentsTest.set_numeric_flag_double_vm fails on some locales (again)

[Ioi Lam]

Please review this trivial fix for ArgumentsTest.set_numeric_flag_double_vm. If an input string is only partially accepted by `strtod()` under the current locale , `end` will point to a non-empty character. In this case, the test should treat the input as invalid.

Thanks to @tstuefe for providing the test scenario.

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Commit messages:
 - 8285279: ArgumentsTest.set_numeric_flag_double_vm fails on some locales (again)

Changes: https://git.openjdk.java.net/jdk/pull/8386/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8386&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285279
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8386.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8386/head:pull/8386

PR: https://git.openjdk.java.net/jdk/pull/8386