The cause of your browser crash is not the overcommit, is simply the fact that you have not enough memory. If you disable overcommit (something you can do on Linux) you would the same crash earlier, before you allocated (not necessary used) 100% of your RAM (because really no software handles the dynamic memory fail condition, i.e. malloc returning null, that you can't handle reasonably).
Null pointers are not a mistake, how do you signal the absence of a value otherwise? How do you signal the failure of a function that returns a pointer without having to return a struct with a pointer and an error code (which is inefficient since the return value doesn't fit a single register)? null makes a perfect sense to be used as a value to signal "this pointer doesn't point to something valid".
Microsoft saying that fork() was a mistake... well, of course, because Windows doesn't have it. fork was a good idea and that is the reason why it's still used these days. Of course nowadays there are evolution, in Linux there is the clone system call (fork is deprecated and still there for compatibility reasons, the glibc fork is implemented with the clone system call). But the concept of creating a process by cloning the resources of the parent is something that to me always seamed very elegant to me.
In reality fork is something that (if I remember correctly, I don't have that much experience in programming in Windows) doesn't exist on Windows, and the only way to create a new process of the same program is to launch the executable, and pass the parameters from the command line, that is not that great for efficiency at all, and also can have its problems (for example the executable was deleted, renamed, etc while the program was running). Also in Windows there is neither the concept of exec, tough I think it can be emulated in software (while fork can't).
To me it makes perfect sense to separate the concept of creating a new process (fork/clone) and loading an executable from disk (exec). It gives a lot of flexibility, at a cost that is not that high (and there are alternatives to avoid it, such as vfork or variations of the clone system call, or directly higher level API such as posix_spawn).