Added missing null terminator when adding characters to the string, otherwise garbage values are read (often zeros which are equal to '\0', but not every time).
This error results in random characters appearing in the text box every one in a while:
```
asdfg??? ll??
```
It is corrected with the proposed fix.
This problem was observed by my student, Gonzalo Rivera Lazo.
Previous GetKeyPressed() method was actually returning unicode codepoints equivalent values instead of the key-code of the pressed key. So, it has been replaced by GetCharPressed(), returning unicode codepoints and GetKeyPressed() now returns key-codes.
* Fixed#1455
Fixed writing out of array bounds
Adjusted FPS comment to match value
Deleted unused function at the end, which has never been in use in the history of this file
* Fixed#1455
Readded the function
