quote infer result dir shell variable

Summary:
public
closes #209

Reviewed By: martinoluca

Differential Revision: D2765557

fb-gh-sync-id: c5b54c1

infer/lib/xcode_wrappers/clang

@@ -9,7 +9,7 @@ export FCP_CLANG_COMPILER="${CLANG_WRAPPERS_PATH%/}/clang_wrapper$XX";
 export FCP_RESULTS_DIR="${INFER_RESULTS_DIR}";
 export FCP_USE_STD_CLANG_CMD="1";
 
-if [ -z $INFER_RESULTS_DIR ]; then
+if [ -z "$INFER_RESULTS_DIR" ]; then
     # this redirects to the compiler without adding any FCP flag
     # this is because xcode requires message category info from the compiler
     # and invokes it without any env var set.