fix(string): strcat does not null-terminate the result

[kudasaixc]

strcat copies the bytes of src after dest but never writes the trailing \0. Any later read of the concatenated string (strlen, terminal_writestring, ...) runs past the end of the buffer into whatever garbage follows, until a stray zero byte is found. The fix writes the terminator before returning.

[kudasaixc]

strcat copies src after dest but never writes the terminating \0:

char dst[64];
dst[0] = '\0';
strcat(dst, "hello");
strcat(dst, "world");
terminal_writestring(dst);   // expected "helloworld"

Because the result is not terminated, terminal_writestring (which relies on strlen) keeps reading bytes past "helloworld" until it happens to hit a stray zero, leaking whatever stack/heap memory sits after the buffer onto the screen, and reading out of bounds if no zero is found.

It also breaks the second strcat: the offset walk while (dest[a] != '\0') a++; can run past the intended end for the same reason. Fix writes dest[a + b] = '\0'; before returning.

[im-nymii]

LGTM !