BUG/MINOR: systemd: check return value of calloc()

The wrapper is not the best reliable thing in the universe, so start
by adding at least the minimum expected controls :-/

To be backported to 1.5 and 1.6.
diff --git a/src/haproxy-systemd-wrapper.c b/src/haproxy-systemd-wrapper.c
index 84d2e17..15c48ca 100644
--- a/src/haproxy-systemd-wrapper.c
+++ b/src/haproxy-systemd-wrapper.c
@@ -77,11 +77,17 @@
 
 	pid = fork();
 	if (!pid) {
-		/* 3 for "haproxy -Ds -sf" */
-		char **argv = calloc(4 + main_argc + nb_pid + 1, sizeof(char *));
+		char **argv;
 		int i;
 		int argno = 0;
 
+		/* 3 for "haproxy -Ds -sf" */
+		argv = calloc(4 + main_argc + nb_pid + 1, sizeof(char *));
+		if (!argv) {
+			fprintf(stderr, SD_NOTICE "haproxy-systemd-wrapper: failed to calloc(), please try again later.\n");
+			exit(1);
+		}
+
 		reset_signal_handler();
 		locate_haproxy(haproxy_bin, 512);
 		argv[argno++] = haproxy_bin;