Bug 1296164 (part 2) - Use [must_use] on nsIFile.open{NSPR,ANSI}FileDesc(). r=froydnj.

The patch also changes RemoteOpenFileChild::OpenNSPRFileDesc() so that it
cannot succeed with a null fd, so that checking just the return value is
sufficient.

xpcom/base/nsGZFileWriter.cpp

@@ -45,7 +45,6 @@ nsGZFileWriter::Init(nsIFile* aFile)
 
   FILE* file;
   nsresult rv = aFile->OpenANSIFileDesc(mMode == Create ? "wb" : "ab", &file);
-
   if (NS_WARN_IF(NS_FAILED(rv))) {
     return rv;
   }