Handle crash when submitting an injected form on an "image page".

Source/core/platform/network/FormDataBuilder.cpp

Index: Source/core/platform/network/FormDataBuilder.cpp
diff --git a/Source/core/platform/network/FormDataBuilder.cpp b/Source/core/platform/network/FormDataBuilder.cpp
index 3dbcd3af89cf225a268d6f67998a739e9c3916d5..b2e62429dd4dc6804ccf891dfa19705f428c8eab 100644
--- a/Source/core/platform/network/FormDataBuilder.cpp
+++ b/Source/core/platform/network/FormDataBuilder.cpp
@@ -91,6 +91,13 @@ WTF::TextEncoding FormDataBuilder::encodingFromAcceptCharset(const String& accep
             return encoding;
     }
 
+    if (document->inputEncoding().isEmpty()) {
+        if (document->defaultCharset().isEmpty())
+            return WTF::UTF8Encoding();
+
+        return document->defaultCharset();
+    }
+
     return document->inputEncoding();
 }