lang/spidermonkey170: unbreak with clang 4.0

Fix quoting failure handling in JSOP_CALLPROP and JSOP_STRING.

jsopcode.cpp:6070:45: error: ordered comparison between pointer and zero ('char *' and 'int')

return QuoteString(&sprinter, s, quote) >= 0;
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~

PR: 215861
Regressed by: https://github.com/llvm-mirror/clang/commit/4b6ad14285f3
Obtained from: upstream (only the specific fix)
Approved by: portmgr blanket
MFH: 2017Q1 (changes existing behavior)