Enhance memory safety and buffer initialization for stability

Describe

I found potential memory safety and stability issues in the kilo codebase:

• CWE-476 (NULL Pointer Dereference):

  In editorUpdateSyntax, editorRowAppendString, and editorOpen, memory allocation results (via realloc or malloc) are not properly checked for NULL.

  This could lead to memory leaks or crashes if an allocation fails.

• CWE-457 (Use of Uninitialized Variable):

  In getCursorPosition, the buffer buf is not explicitly initialized, which could result in undefined behavior.

Expected behavior

• Memory allocation failures (e.g., realloc, malloc) should be properly handled and should not overwrite original pointers.
• Buffers should be initialized before use to avoid undefined behavior.

Actual behavior

• In multiple functions, the return values of realloc or malloc are not checked before dereferencing.
• In getCursorPosition, buf may contain uninitialized data before valid input is read.

How to Reproduce

• Simulate low-memory conditions (e.g., ulimit -v 10000 on Linux) and observe crashes or instability.