Improve memory safety in the DOS Drive Cache class

- Fix Bitu printf format type
- Check a pointer prior to dereferencing it
- Prevent writing one-beyond the last index
- Replace strcpy with with helper safe_strcpy, provided by @dreamer - thank you!
- Replace strcat with strncat
- Add constructor intializers for scalars and arrays
- Initialize and replace 0-value pointers with nullptr
- Pass in the buffer length when strncpy'ing into a function variable

include/support.h

@@ -21,9 +21,10 @@
 #define DOSBOX_SUPPORT_H
 
 #include <algorithm>
+#include <cstdio>
+#include <ctype.h>
 #include <string.h>
 #include <string>
-#include <ctype.h>
 #ifndef DOSBOX_DOSBOX_H
 #include "dosbox.h"
 #endif
@@ -33,6 +34,27 @@
 #define strncasecmp(a,b,n) _strnicmp(a,b,n)
 #endif
 
+/// Copy a string into C array
+///
+/// This function copies string pointed by src to fixed-size buffer dst.
+/// At most N bytes from src are copied, where N is size of dst.
+/// If exactly N bytes are copied, then terminating null byte is put
+/// into buffer, thus buffer overrun is prevented.
+///
+/// Function returns pointer to buffer to be compatible with std::strcpy.
+///
+/// Usage:
+///
+///     char buffer[2];
+///     safe_strcpy(buffer, "abc");
+///     // buffer is filled with "a"
+
+template<size_t N>
+char * safe_strcpy(char (& dst)[N], const char * src) noexcept {
+    snprintf(dst, N, "%s", src);
+    return & dst[0];
+}
+
 #define safe_strncpy(a,b,n) do { strncpy((a),(b),(n)-1); (a)[(n)-1] = 0; } while (0)
 
 #ifdef HAVE_STRINGS_H