BUG/MINOR: proxy: Missing calloc return value check in proxy_defproxy_cpy

A memory allocation failure happening in proxy_defproxy_cpy while
copying the default compression options would have resulted in a crash.
This function is called for every new proxy found while parsing the
configuration.

It was raised in GitHub issue #1233.
It could be backported to all stable branches.
diff --git a/src/proxy.c b/src/proxy.c
index 079280c..0e96571 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -1717,6 +1717,10 @@
 	/* default compression options */
 	if (defproxy->comp != NULL) {
 		curproxy->comp = calloc(1, sizeof(*curproxy->comp));
+		if (!curproxy->comp) {
+			memprintf(errmsg, "proxy '%s': out of memory for default compression options", curproxy->id);
+			return 1;
+		}
 		curproxy->comp->algos = defproxy->comp->algos;
 		curproxy->comp->types = defproxy->comp->types;
 	}