Don't crash on --print-variables when there are no variables

Apparently g_hash_table_foreach doesn't check for NULL input, so make
sure we don't call it to print the variables if the variable list is
empty.

Freedesktop #54721

check/Makefile.am

@@ -49,4 +49,5 @@ EXTRA_DIST = \
 	non-l-required.pc \
 	circular-1.pc \
 	circular-2.pc \
-	circular-3.pc
+	circular-3.pc \
+	no-variables.pc

check/check-print-options

@@ -11,7 +11,10 @@ run_test --version
 RESULT=1.0.0
 run_test --modversion simple
 
-# --print-variables
+# --print-variables, make sure having no variables doesn't crash
+RESULT=""
+run_test --print-variables no-variables
+
 RESULT="exec_prefix
 prefix
 libdir

check/no-variables.pc

@@ -0,0 +1,3 @@
+Name: No Variables
+Description: Test package having no variables to check --print-variables
+Version: 4.2

main.c

@@ -580,9 +580,10 @@ main (int argc, char **argv)
       while (tmp != NULL)
         {
           Package *pkg = tmp->data;
-          g_hash_table_foreach(pkg->vars,
-                               &print_hashtable_key,
-                               NULL);
+          if (pkg->vars != NULL)
+            g_hash_table_foreach(pkg->vars,
+                                 &print_hashtable_key,
+                                 NULL);
           tmp = g_slist_next (tmp);
           if (tmp) printf ("\n");
         }