Improve error message when registering empty bundles

Summary:
This change lets `registerBundle(bundleId, file)` throw an exception
when the file is empty, improving on the current behavior of an
eventual SIGABRT saying "MAP_FAILED: Invalid argument"

Reviewed By: ridiculousfish

Differential Revision: D16451938

fbshipit-source-id: b8b2d0bfed476319c379122fad59a5bf0a8c813b

ReactCommon/cxxreact/JSBigString.cpp

@@ -110,6 +110,9 @@ static off_t maybeRemap(char *data, size_t size, int fd) {
 #endif // WITH_FBREMAP
 
 const char *JSBigFileString::c_str() const {
+  if (m_size == 0) {
+    return "";
+  }
   if (!m_data) {
     m_data =
       (const char *) mmap(0, m_size, PROT_READ, MAP_PRIVATE, m_fd, m_mapOff);

ReactCommon/jsiexecutor/jsireact/JSIExecutor.cpp

@@ -161,6 +161,10 @@ void JSIExecutor::registerBundle(
     bundleRegistry_->registerBundle(bundleId, bundlePath);
   } else {
     auto script = JSBigFileString::fromPath(bundlePath);
+    if (script->size() == 0) {
+      throw std::invalid_argument(
+          "Empty bundle registered with ID " + tag + " from " + bundlePath);
+    }
     runtime_->evaluateJavaScript(
         std::make_unique<BigStringBuffer>(std::move(script)),
         JSExecutor::getSyntheticBundlePath(bundleId, bundlePath));