raygui: fix buffer overflow crash in TextBox when string is smaller than textSize

[shdynila]

Description

Fixes a heap corruption/buffer overflow crash when calling raygui.TextBox with a string that is shorter than the requested textSize (e.g., an empty string "" with a textSize of 25).

Root Cause

In Go, bs := []byte(*text) creates a byte slice whose size is bound to the string's current length, rather than the requested textSize.
If len(bs) is smaller than textSize, the C function GuiTextBox receives a pointer to a small Go-allocated buffer but expects it to be at least textSize bytes. When typing a character, GuiTextBox writes out-of-bounds, corrupting the Go heap and causing access violations / segmentation faults.

Solution

1. Ensured Minimum Buffer Capacity: If the string slice bs is smaller than textSize, it is re-allocated to have a length of at least textSize (newBs := make([]byte, textSize); copy(newBs, bs)).
2. Migrated to C-Heap Allocation (C.CString): Instead of passing a Go garbage-collected slice pointer directly to C (&bs[0]), which violates cgo pointer-passing rules for mutable buffers, we now copy it safely to the C-heap using C.CString(string(bs)).
3. Safe Memory Cleanup & Synchronization: We defer freeing the C-allocated memory using C.free and successfully copy the updated string back into Go memory with *text = C.GoString(ctext) after GuiTextBox executes.

[gen2brain]

Thanks, I am merging this. Note that raygui in this state is contributed; before, there was a plain Go code that just called into the raylib bindings, mimicking what the C API did. But, maintaining and following upstream was an issue, so I accepted the CGo solution that worked for users. Perhaps, introducing that again in the age of AI, where it is not that difficult to follow, is again a good idea.