Fix clang compilation error in jscre.

Fix clang compilation error in jscre.

../../dart/runtime/third_party/jscre/pcre_internal.h:412:5: error: implicit conversion of NULL constant to 'int' [-Werror,-Wnull-conversion]
    ASSERT(isBracketStartOpcode(*opcodePtr) || *opcodePtr == OP_ALT);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../dart/runtime/third_party/jscre/pcre_internal.h:86:72: note: expanded from macro 'ASSERT'
#define ASSERT(x) if (!(x)) *(reinterpret_cast<volatile int*>(NULL)) = NULL

BUG=http://dartbug.com/19324
R=iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=39276

[Ivan Posva, 2014-08-14 22:58:54]

LGTM -ip

[rmacnak, 2014-08-14 23:19:16]

Committed patchset #1 manually as 39276.