make system.* functions support UTF8 filenames (#1042)

* make system.* functions support UTF8 filenames
* move utfconv.h into ifdef guard
* fix wrong null check

src/utfconv.h

@@ -0,0 +1,57 @@
+#ifndef MBSEC_H
+#define MBSEC_H
+
+#ifdef _WIN32
+
+#include <stdlib.h>
+#include <windows.h>
+
+#define UTFCONV_ERROR_INVALID_CONVERSION "Input contains invalid byte sequences."
+
+LPWSTR utfconv_utf8towc(const char *str) {
+  LPWSTR output;
+  int len;
+
+  // len includes \0
+  len = MultiByteToWideChar(CP_UTF8, 0, str, -1, NULL, 0);
+  if (len == 0)
+    return NULL;
+
+  output = (LPWSTR) malloc(sizeof(WCHAR) * len);
+  if (output == NULL)
+    return NULL;
+
+  len = MultiByteToWideChar(CP_UTF8, 0, str, -1, output, len);
+  if (len == 0) {
+    free(output);
+    return NULL;
+  }
+
+  return output;
+}
+
+char *utfconv_wctoutf8(LPCWSTR str) {
+  char *output;
+  int len;
+
+  // len includes \0
+  len = WideCharToMultiByte(CP_UTF8, 0, str, -1, NULL, 0, NULL, NULL);
+  if (len == 0)
+    return NULL;
+
+  output = (char *) malloc(sizeof(char) * len);
+  if (output == NULL)
+    return NULL;
+
+  len = WideCharToMultiByte(CP_UTF8, 0, str, -1, output, len, NULL, NULL);
+  if (len == 0) {
+    free(output);
+    return NULL;
+  }
+
+  return output;
+}
+
+#endif
+
+#endif