some more boundary checking

src/refresh/files/jpeg.c

@@ -90,7 +90,7 @@ LoadJPG(char *origname, byte **pic, int *width, int *height)
 
 	/* Add the extension */
 	len = strlen(filename);
-	if (strcmp(filename + len - 4, ".jpg"))
+	if (len >= 4 && strcmp(filename + len - 4, ".jpg"))
 	{
 		strncat(filename, ".jpg", sizeof(filename) - 1 - len);
 	}

src/refresh/files/pcx.c

@@ -42,8 +42,8 @@ LoadPCX(char *origname, byte **pic, byte **palette, int *width, int *height)
 	filename[sizeof(filename) - 1] = '\0';
 
 	/* Add the extension */
-	filelen = strlen(origname);
-	if (strcmp(filename + filelen - 4, ".pcx"))
+	filelen = strlen(filename);
+	if (filelen >= 4 && strcmp(filename + filelen - 4, ".pcx"))
 	{
 		strncat(filename, ".pcx", sizeof(filename) - 1 - filelen);
 	}

src/refresh/files/tga.c

@@ -54,8 +54,8 @@ LoadTGA(char *origname, byte **pic, int *width, int *height)
 	name[sizeof(name) - 1] = '\0';
 
 	/* Add the extension */
-	len = strlen(origname);
-	if (strcmp(name + len - 4, ".tga"))
+	len = strlen(name);
+	if (len >= 4 && strcmp(name + len - 4, ".tga"))
 	{
 		strncat(name, ".tga", sizeof(name) - 1 - len);
 	}

src/refresh/files/wal.c

@@ -40,7 +40,7 @@ LoadWal(char *origname)
 
 	/* Add the extension */
 	len = strlen(name);
-	if (strcmp(name + len - 4, ".wal"))
+	if (len >= 4 && strcmp(name + len - 4, ".wal"))
 	{
 		strncat(name, ".wal", sizeof(name) - 1 - len);
 	}