Fix -Warray-bounds warnings

[phikal]

I have GCC 12 on my system, and compiling neatvi triggered these warnings

cc -c -Wall -O2 reg.c
reg.c: In function ‘reg_putraw’:
reg.c:21:18: warning: array subscript [-128, 255] is outside array bounds of ‘char *[256]’ [-Warray-bounds]
   21 |         free(bufs[tolower(c)]);
      |              ~~~~^~~~~~~~~~~~
reg.c:6:14: note: while referencing ‘bufs’
    6 | static char *bufs[256];
      |              ^~~~
reg.c:22:13: warning: array subscript [-128, 255] is outside array bounds of ‘char *[256]’ [-Warray-bounds]
   22 |         bufs[tolower(c)] = buf;
      |         ~~~~^~~~~~~~~~~~
reg.c:6:14: note: while referencing ‘bufs’
    6 | static char *bufs[256];
      |              ^~~~

The issue is that tolower does not necessarily return a value between 0 and 255, though this is to my understanding the implicit understanding in the code?