content_encoding: make Curl_all_content_encodings allocless

- Fixes a memory leak pointed out by Coverity
- Also found by OSS-Fuzz: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=63947
- Avoids unncessary allocations

Follow-up ad051e1cbe

Closes #12289

lib/content_encoding.h

@@ -27,7 +27,7 @@
 
 struct Curl_cwriter;
 
-char *Curl_all_content_encodings(void);
+void Curl_all_content_encodings(char *buf, size_t blen);
 
 CURLcode Curl_build_unencoding_stack(struct Curl_easy *data,
                                      const char *enclist, int is_transfer);