debListParser: ParseDepends: Only query native arch if needed

This makes the code parsing architecture lists slower, but on
the other hand, improves the more generic case of reading
dependencies from Packages files.

apt-pkg/deb/deblistparser.cc

@@ -562,8 +562,8 @@ const char *debListParser::ParseDepends(const char *Start,const char *Stop,
    Package.assign(Start,I - Start);
 
    // We don't want to confuse library users which can't handle MultiArch
-   string const arch = _config->Find("APT::Architecture");
    if (StripMultiArch == true) {
+      string const arch = _config->Find("APT::Architecture");
       size_t const found = Package.rfind(':');
       if (found != string::npos &&
 	  (strcmp(Package.c_str() + found, ":any") == 0 ||
@@ -609,6 +609,7 @@ const char *debListParser::ParseDepends(const char *Start,const char *Stop,
 
    if (ParseArchFlags == true)
    {
+      string const arch = _config->Find("APT::Architecture");
       APT::CacheFilter::PackageArchitectureMatchesSpecification matchesArch(arch, false);
 
       // Parse an architecture