Change char->free to auto_char char for autocleanup

Replace `gchar` and `g_free` to `auto_gchar`
Correct certain  `char` functions/variables to `gchar`

Related to #1819.

Edited by @jubalh.

src/config/color.c

@@ -35,6 +35,7 @@
  */
 
 #include "config.h"
+#include "common.h"
 
 #include <stdlib.h>
 #include <string.h>
@@ -508,10 +509,9 @@ color_pair_cache_hash_str(const char* str, color_profile profile)
     int fg = color_hash(str, profile);
     int bg = -1;
 
-    char* bkgnd = theme_get_bkgnd();
+    auto_char char* bkgnd = theme_get_bkgnd();
     if (bkgnd) {
         bg = find_col(bkgnd, strlen(bkgnd));
-        free(bkgnd);
     }
 
     return _color_pair_cache_get(fg, bg);