fix: added null check & type checking

[arteevraina]

• Checks if *x is nullptr.

Refer: #1599

[certik]

Can you check that this PR now reports an error for that issue instead of a segfault?

[arteevraina]

Can you check that this PR now reports an error for that issue instead of a segfault?

Yes, It gives an error message now instead of segfault for #1599

(lp) arteevraina@Arteevs-MacBook-Air lpython % src/bin/lpython src/a.py                                         
code generation error: Internal error: x is nullptr


Note: if any of the above error or warning messages are not clear or are lacking
context please report it to us (we consider that a bug that must be fixed).

[certik]

Very good, let's see if the tests pass.

[certik]

Thanks!