subprocess: Proper implementation of wait() on Windows

This commit makes sure:
1. WaitForSingleObject returns with expected
code before proceeding.
2. Process handle is properly closed.

Github-Pull: arun11299/cpp-subprocess#116
Rebased-From: 625a8775791e62736f20f3fa3e6cc4f1b24aa89a

Author: lunacd

src/util/subprocess.h

@@ -1061,11 +1061,18 @@ inline int Popen::wait() noexcept(false)
 #ifdef __USING_WINDOWS__
   int ret = WaitForSingleObject(process_handle_, INFINITE);
 
+  // WaitForSingleObject with INFINITE should only return when process has signaled
+  if (ret != WAIT_OBJECT_0) {
+    throw OSError("Unexpected return code from WaitForSingleObject", 0);
+  }
+
   DWORD dretcode_;
 
   if (FALSE == GetExitCodeProcess(process_handle_, &dretcode_))
       throw OSError("Failed during call to GetExitCodeProcess", 0);
 
+  CloseHandle(process_handle_);
+
   return (int)dretcode_;
 #else
   int ret, status;