Fix the build.

tools/flags/SkCommandLineFlags.cpp

Index: tools/flags/SkCommandLineFlags.cpp
diff --git a/tools/flags/SkCommandLineFlags.cpp b/tools/flags/SkCommandLineFlags.cpp
index 7420d2643e9391ca904220319d448b88bf6306dc..ed8db8e7055fed0c9dccbd786388eb53338dfc61 100644
--- a/tools/flags/SkCommandLineFlags.cpp
+++ b/tools/flags/SkCommandLineFlags.cpp
@@ -22,6 +22,9 @@ bool SkFlagInfo::CreateStringFlag(const char* name, const char* shortName,
 void SkFlagInfo::SetDefaultStrings(SkCommandLineFlags::StringArray* pStrings,
                                    const char* defaultValue) {
     pStrings->reset();
+    if (NULL == defaultValue) {
+        return;
+    }
     // If default is "", leave the array empty.
     size_t defaultLength = strlen(defaultValue);
     if (defaultLength > 0) {