Fix ScopedSignalHandler

I4ae2d82fdfdd58cf8ef1dfb6c401cef4ba9d3e88 broke memunreachable_test
by replacing signal_ with signal.

Test: memunreachable_test
Change-Id: I8148321e3a94530867106708a33b5b63ed867380
diff --git a/ScopedSignalHandler.h b/ScopedSignalHandler.h
index f62f368..ff53fad 100644
--- a/ScopedSignalHandler.h
+++ b/ScopedSignalHandler.h
@@ -37,7 +37,7 @@
 
   template <class F>
   void install(int signal, F&& f) {
-    if (signal != -1) MEM_LOG_ALWAYS_FATAL("ScopedSignalHandler already installed");
+    if (signal_ != -1) MEM_LOG_ALWAYS_FATAL("ScopedSignalHandler already installed");
 
     handler_ = SignalFn(std::allocator_arg, allocator_,
                         [=](int signal, siginfo_t* si, void* uctx) { f(*this, signal, si, uctx); });