RFR (XXS): 8246557: test_os_linux.cpp uses NULL instead of MAP_FAILED to check for failed mmap call

[Thomas Schatzl]

Hi all,

   can I have reviews for this (trivial?) change that fixes wrong 
detection logic for the mmap calls in the test_os_linux gtests?

Instead of

    ASSERT_TRUE(mapping != NULL) << " mmap failed, mapping_size = " << 
mapping_size;

it should use

    ASSERT_TRUE(mapping != MAP_FAILED) << " mmap failed, mapping_size = 
" << mapping_size;

since MAP_FAILED = (void*)-1.

All other uses of mmap seem to be okay.

CR:
https://bugs.openjdk.java.net/browse/JDK-8246557
Webrev:
http://cr.openjdk.java.net/~tschatzl/8246557/webrev/
Testing:
hs-tier1

Thanks,
   Thomas