free: instead of Curl_safefree()

Since we just started make use of free(NULL) in order to simplify code,
this change takes it a step further and:

- converts lots of Curl_safefree() calls to good old free()
- makes Curl_safefree() not check the pointer before free()

The (new) rule of thumb is: if you really want a function call that
frees a pointer and then assigns it to NULL, then use Curl_safefree().
But we will prefer just using free() from now on.

lib/curl_multibyte.c

@@ -47,7 +47,8 @@ wchar_t *Curl_convert_UTF8_to_wchar(const char *str_utf8)
       if(str_w) {
         if(MultiByteToWideChar(CP_UTF8, 0, str_utf8, -1, str_w,
                                str_w_len) == 0) {
-          Curl_safefree(str_w);
+          free(str_w);
+          return NULL;
         }
       }
     }
@@ -68,7 +69,8 @@ char *Curl_convert_wchar_to_UTF8(const wchar_t *str_w)
       if(str_utf8) {
         if(WideCharToMultiByte(CP_UTF8, 0, str_w, -1, str_utf8, str_utf8_len,
                                NULL, FALSE) == 0) {
-          Curl_safefree(str_utf8);
+          free(str_utf8);
+          return NULL;
         }
       }
     }