tests: Prevent malloc with size 0

Found by clang-tidy's "clang-analyzer-optin.portability.UnixAPI" check.

Signed-off-by: Tim Wiederhake <twiederh@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

tests/commandhelper.c

@@ -156,6 +156,9 @@ static int printEnvironment(FILE *log)
     for (length = 0; environ[length]; length++) {
     }
 
+    if (length == 0)
+        return 0;
+
     if (!(newenv = malloc(sizeof(*newenv) * length)))
         return -1;