http: silence compile-time warnings without USE_NGHTTP2

Error: CLANG_WARNING:
lib/http.c:173:16: warning: Value stored to 'http' during its initialization is never read

Error: COMPILER_WARNING:
lib/http.c: scope_hint: In function ‘http_disconnect’
lib/http.c:173:16: warning: unused variable ‘http’ [-Wunused-variable]