fix heap-buffer-overflow (#957)

Prevent incrementing passed a `\0` in our format string.

Co-authored-by: Michael Grunder <michael.grunder@gmail.com>
Co-authored-by: Kristján Valur Jónsson <sweskman@gmail.com>

hiredis.c

@@ -485,6 +485,8 @@ int redisvFormatCommand(char **target, const char *format, va_list ap) {
 
             touched = 1;
             c++;
+            if (*c == '\0')
+                break;
         }
         c++;
     }