RFR: 8194154: JDK crashes parsing path string contains '//' on linux

[yumin qi]

Hi,

  Please review the fix (extra small) for:
  bug 8194154: https://bugs.openjdk.java.net/browse/JDK-8194154
  webrev: http://cr.openjdk.java.net/~minqi/8194154/

  Summary: canonicalize will check and fold double (or more) slashes, but
in function collapsible(char*) and splitNames(char*, char**), it failed to
process strings like "//". This results in the former does not give a
correct number of substrings and the later does not give a correct array of
substrings. The fix add a check if the character is '/' after a '/'.

    The test case added will crash without the fix.

 Thanks
 Yumin