BUG/MINOR: tools: make str2sa_range() report unresolvable addresses

If an environment variable is used in an address, and is not set, it's
silently considered as ":" or "0.0.0.0:0" which is not correct as it
can hide environment issues and lead to unexpected behaviours. Let's
report this case when it happens.

This fix should be backported to 1.5.
diff --git a/src/standard.c b/src/standard.c
index 58cc5de..7964b88 100644
--- a/src/standard.c
+++ b/src/standard.c
@@ -782,6 +782,11 @@
 		goto out;
 	}
 
+	if (!*str2) {
+		memprintf(err, "'%s' resolves to an empty address (environment variable missing?)\n", str);
+		goto out;
+	}
+
 	memset(&ss, 0, sizeof(ss));
 
 	if (strncmp(str2, "unix@", 5) == 0) {