v.in.dwg: avoid src and dst overlap in sprintf

Currently, one instance of sprintf has same src and dst pointers,
probably to append new information to the buffer. But, having
overlapping src and dst pointers is undefined behavior.

In the new set of changes, code is refactored to automatically
start writing new data at the end of previous data. snprintf
is also used over sprintf to be wary of the fact that buffer
size is limited to 2000.

Since, snprintf automatically adds a null character to the end,
to find the remaining buffer space, we subtract existing buffer
length as well as null character to find maximum possible space
to write in the buffer. This translates to '2000 - buflen - 1'.

Signed-off-by: Mohan Yelugoti <ymdatta.work@gmail.com>

vector/v.in.dwg/main.c

@@ -137,8 +137,11 @@ int main(int argc, char *argv[])
     if (!adInitAd2(path, &initerror)) {
         sprintf(buf, _("Unable to initialize OpenDWG Toolkit, error: %d: %s."),
                 initerror, adErrorStr(initerror));
-        if (initerror == AD_UNABLE_TO_OPEN_INIT_FILE)
-            sprintf(buf, _("%s Cannot open %s"), buf, path);
+        size_t buflen = strlen(buf);
+        if (initerror == AD_UNABLE_TO_OPEN_INIT_FILE) {
+            snprintf(buf + buflen, 2000 - buflen - 1, (" Cannot open %s"),
+                     path);
+        }
         G_fatal_error(buf);
     }
     adSetupDwgRead();