[yaml2obj] Fixing opening empty yaml files.

[plotfi]

Summary

Essentially echo "" | yaml2obj crashes. This patch attempts to trim whitespace and determine if the yaml string in the file is empty or not.

[compnerd]

Please check the error message is printed in the case the YAML is empty.

[pcc]

Essentially echo "" | yaml2obj crashes.

Why is this the right place for the fix then? Isn't it a bug in yaml::Input?

[grimar]

FYI: https://reviews.llvm.org/D38036

test/tools/yaml2obj/empty.yaml

+# RUN: echo "" | not yaml2obj
+# RUN: echo -n "" | not yaml2obj
+# RUN: echo " " | not yaml2obj
+# RUN: echo "  " | not yaml2obj

tools/yaml2obj/yaml2obj.cpp

int main(int argc, char **argv) {
   if (EC)
     error("yaml2obj: Error opening '" + OutputFilename + "': " + EC.message());
 
   ErrorOr<std::unique_ptr<MemoryBuffer>> Buf =
       MemoryBuffer::getFileOrSTDIN(Input);
   if (!Buf)
     return 1;
 
-  yaml::Input YIn(Buf.get()->getBuffer());
+  StringRef Buffer = Buf.get()->getBuffer();
+  if (Buffer.trim().size() == 0)
+    error("yaml2obj: Error opening '" + Input + "': Empty File.");
+
+  yaml::Input YIn(Buffer);
 
   int Res = convertYAML(YIn, Out->os());
   if (Res == 0)
     Out->keep();
 
   Out->os().flush();
   return Res;
 }