Fixed the indexing of filename_t past its end in DD_SetConfigFile.

Also, fixed dangerous misplaced parentheses in the same function.

Danij: I am officially worried about the usage of FILENAME_T_MAXLEN.
You should check each instance where it's used and check if, in fact, 
you should use FILENAME_T_LASTINDEX.

doomsday/engine/api/dd_types.h

@@ -119,9 +119,11 @@ typedef unsigned short  thid_t;
 typedef unsigned char   byte;
 typedef double          timespan_t;
 
-#define FILENAME_T_MAXLEN (256)
+#define FILENAME_T_MAXLEN 256
 typedef char            filename_t[FILENAME_T_MAXLEN];
 
+#define FILENAME_T_LASTINDEX 255
+
 typedef struct directory_s {
     int             drive;
     filename_t      path;

doomsday/engine/portable/src/dd_main.c

@@ -263,16 +263,16 @@ int DD_AddAutoData(boolean loadFiles)
 void DD_SetConfigFile(const char* file)
 {
     strncpy(configFileName, file, FILENAME_T_MAXLEN);
-    configFileName[FILENAME_T_MAXLEN] = '\0';
+    configFileName[FILENAME_T_LASTINDEX] = '\0';
 
     Dir_FixSlashes(configFileName, FILENAME_T_MAXLEN);
 
     strncpy(bindingsConfigFileName, configFileName, FILENAME_T_MAXLEN);
-    bindingsConfigFileName[FILENAME_T_MAXLEN] = '\0';
+    bindingsConfigFileName[FILENAME_T_LASTINDEX] = '\0';
 
     strncpy(bindingsConfigFileName + strlen(bindingsConfigFileName) - 4,
-            "-bindings.cfg", FILENAME_T_MAXLEN - strlen(bindingsConfigFileName - 4));
-    bindingsConfigFileName[FILENAME_T_MAXLEN] = '\0';
+            "-bindings.cfg", FILENAME_T_MAXLEN - strlen(bindingsConfigFileName) - 4);
+    bindingsConfigFileName[FILENAME_T_LASTINDEX] = '\0';
 }
 
 /**