http2: fix resource leaks in set_transfer_url()

http2: fix resource leaks in set_transfer_url()

... detected by Coverity:

Error: RESOURCE_LEAK (CWE-772):
lib/http2.c:480: alloc_fn: Storage is returned from allocation function "curl_url". [Note: The source code implementation of the function has been overridden by a builtin model.]
lib/http2.c:480: var_assign: Assigning: "u" = storage returned from "curl_url()".
lib/http2.c:486: noescape: Resource "u" is not freed or pointed-to in "curl_url_set". [Note: The source code implementation of the function has been overridden by a builtin model.]
lib/http2.c:488: leaked_storage: Variable "u" going out of scope leaks the storage it points to.

Error: RESOURCE_LEAK (CWE-772):
lib/http2.c:480: alloc_fn: Storage is returned from allocation function "curl_url". [Note: The source code implementation of the function has been overridden by a builtin model.]
lib/http2.c:480: var_assign: Assigning: "u" = storage returned from "curl_url()".
lib/http2.c:493: noescape: Resource "u" is not freed or pointed-to in "curl_url_set". [Note: The source code implementation of the function has been overridden by a builtin model.]
lib/http2.c:495: leaked_storage: Variable "u" going out of scope leaks the storage it points to.

Error: RESOURCE_LEAK (CWE-772):
lib/http2.c:480: alloc_fn: Storage is returned from allocation function "curl_url". [Note: The source code implementation of the function has been overridden by a builtin model.]
lib/http2.c:480: var_assign: Assigning: "u" = storage returned from "curl_url()".
lib/http2.c:500: noescape: Resource "u" is not freed or pointed-to in "curl_url_set". [Note: The source code implementation of the function has been overridden by a builtin model.]
lib/http2.c:502: leaked_storage: Variable "u" going out of scope leaks the storage it points to.

Error: RESOURCE_LEAK (CWE-772):
lib/http2.c:480: alloc_fn: Storage is returned from allocation function "curl_url". [Note: The source code implementation of the function has been overridden by a builtin model.]
lib/http2.c:480: var_assign: Assigning: "u" = storage returned from "curl_url()".
lib/http2.c:505: noescape: Resource "u" is not freed or pointed-to in "curl_url_get". [Note: The source code implementation of the function has been overridden by a builtin model.]
lib/http2.c:507: leaked_storage: Variable "u" going out of scope leaks the storage it points to.

Closes #6986