[yaml2obj] Fixing opening empty yaml files.

Essentially echo "" | yaml2obj crashes. This patch attempts to trim whitespace
and determine if the yaml string in the file is empty or not. If the input is
empty then it will not properly print out an error message and return an error
code.

Differential Revision: https://reviews.llvm.org/D59964

A    test/tools/yaml2obj/empty.yaml
M    tools/yaml2obj/yaml2obj.cpp

llvm-svn: 357219

llvm/test/tools/yaml2obj/empty.yaml

@@ -0,0 +1,5 @@
+# RUN: echo "" | not yaml2obj 2>&1 | FileCheck %s
+# RUN: echo -n "" | not yaml2obj 2>&1 | FileCheck %s
+# RUN: echo " " | not yaml2obj 2>&1 | FileCheck %s
+# RUN: echo "  " | not yaml2obj 2>&1 | FileCheck %s
+# CHECK: yaml2obj: Error opening '-': Empty File.

llvm/tools/yaml2obj/yaml2obj.cpp

@@ -86,7 +86,10 @@ int main(int argc, char **argv) {
   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)