BUG/MINOR: proxy: preset the error message pointer to NULL in parse_new_proxy()

As reported by Coverity in issue #1568, a missing initialization of the
error message pointer in parse_new_proxy() may result in displaying garbage
or crashing in case of memory allocation error when trying to create a new
proxy on startup.

This should be backported to 2.4.
diff --git a/src/proxy.c b/src/proxy.c
index c486376..4991d94 100644
--- a/src/proxy.c
+++ b/src/proxy.c
@@ -1839,7 +1839,7 @@
                               const struct proxy *defproxy)
 {
 	struct proxy *curproxy = NULL;
-	char *errmsg;
+	char *errmsg = NULL;
 
 	if (!(curproxy = alloc_new_proxy(name, cap, &errmsg))) {
 		ha_alert("parsing [%s:%d] : %s\n", file, linenum, errmsg);