RFR(S) 8154820 - JDWP: -agentlib:jdwp=help assertion error

[Ioi Lam]

Please review a small fix:

Bug: https://bugs.openjdk.java.net/browse/JDK-8154820
Desc: JDWP: -agentlib:jdwp=help assertion error
Fix: http://cr.openjdk.java.net/~iklam/jdk9/8154820_jdwp_help_exit.v02/


The were two issues:

[1] MAXPATHLEN is really small on Windows (~256), so MAX_MESSAGE_LEN
     was around 1024, but the jdwp help message is about 1600 bytes.
     The fix would expand it to abut 2500 bytes which is more than enough.

[2] The meaning of the "outputMaxLen" parameter to the utf8ToPlatform()
     and utf8FromPlatform() functions was ambiguous. The assert

        UTF_ASSERT(outputMaxLen > len);

     suggest that the trailing zero is included in the buffer size, so
     at most outputMaxLenbytes are written.

     However, the implementation actually would write outputMaxLen+1 bytes
     in the worst case.

     Luckily, this does not seem to be a problem as all calls to
     utf8ToPlatform/utf8FromPlatform allocate way more space than
     needed. The only exception was vprint_message, which chose to pass

       outputMaxLen := sizeof(buf) -1

     So in the worst case, the VM would exit due to the above 
UTF_ASSERT, but
     we would not silently write one byte past the end of the buffer.

     The fix is:
     [a] Clarify the meaning of the outputMaxLen parameter
     [b] Make sure we don't write past the end of the buffer
     [c] Fix vprint_message() to pass in the correct size of the
         buffer to include space for the trailing zero.

Thanks
Ioi