lib: fix uninitialized variable

```
Error: UNINIT (CWE-457):
libeconf-0.4.1/lib/libeconf.c:161: var_decl: Declaring variable "key_file" without initializer.
libeconf-0.4.1/lib/libeconf.c:247: uninit_use_in_call: Using uninitialized value "key_file" when calling "econf_freeFile".
245|     *key_files = calloc(*size, sizeof(econf_file*));
246|     if (*key_files == NULL) {
247|->     econf_freeFile(key_file);
248|       return ECONF_NOMEM;
249|     }
```

Resolves: https://issues.redhat.com/browse/RHEL-24989

Signed-off-by: Iker Pedrosa <[email protected]>

lib/readconfig.c

@@ -29,7 +29,7 @@ econf_err readConfigHistoryWithCallback(econf_file ***key_files,
 {
   const char *suffix, *default_dirs[4] = {NULL, NULL, NULL, NULL};
   char *distfile, *runfile, *etcfile, *cp;
-  econf_file *key_file;
+  econf_file *key_file = NULL;
   econf_err error;
 
   *size = 0;