src: relax assertion in operator[] of MaybeStackBuffer

Setting environment variables on Windows was causing an abort because
the assertion did not account for the null-terminator at the end of the
buffer.

src/node_env_var.cc

@@ -121,7 +121,7 @@ void RealEnvStore::Set(Isolate* isolate,
   node::Utf8Value val(isolate, value);
 
 #ifdef _WIN32
-  if (key.length() > 0 && key[0] == L'=') return;
+  if (key[0] == L'=') return;
 #endif
   uv_os_setenv(*key, *val);
   DateTimeConfigurationChangeNotification(isolate, key);

src/util.h

@@ -351,12 +351,12 @@ class MaybeStackBuffer {
   }
 
   T& operator[](size_t index) {
-    CHECK_LT(index, length());
+    CHECK_LT(index, length() + 1);
     return buf_[index];
   }
 
   const T& operator[](size_t index) const {
-    CHECK_LT(index, length());
+    CHECK_LT(index, length() + 1);
     return buf_[index];
   }