Add and use str2wcstring(string, size_t)

This is just for convenience.

src/builtin_string.cpp

@@ -91,7 +91,7 @@ static const wchar_t *string_get_arg_stdin(wcstring *storage, const io_streams_t
     }
 
     // Split the buffer on the '\n' and return the first part.
-    *storage = str2wcstring(buffer.c_str(), pos);
+    *storage = str2wcstring(buffer, pos);
     buffer.erase(0, pos + 1);
     return storage->c_str();
 }

src/common.cpp

@@ -290,6 +290,11 @@ wcstring str2wcstring(const std::string &in) {
     return str2wcs_internal(in.data(), in.size());
 }
 
+wcstring str2wcstring(const std::string &in, size_t len) {
+    // Handles embedded nulls!
+    return str2wcs_internal(in.data(), len);
+}
+
 char *wcs2str(const wchar_t *in) {
     if (!in) return NULL;
     size_t desired_size = MAX_UTF8_BYTES * wcslen(in) + 1;

src/common.h

@@ -285,6 +285,7 @@ int fgetws2(wcstring *s, FILE *f);
 wcstring str2wcstring(const char *in);
 wcstring str2wcstring(const char *in, size_t len);
 wcstring str2wcstring(const std::string &in);
+wcstring str2wcstring(const std::string &in, size_t len);
 
 /// Returns a newly allocated multibyte character string equivalent of the specified wide character
 /// string.