Fix FS_FOpenFileRead corner case

FS_FOpenFileRead is a fairly mental function that changes its return
behaviour depending on whether or not file is NULL or not. It turns out
in the case where file is NULL, we were returning the wrong value when
the file didn't exist.

code/qcommon/files.c

@@ -1360,10 +1360,17 @@ long FS_FOpenFileRead(const char *filename, fileHandle_t *file, qboolean uniqueF
 		fprintf(missingFiles, "%s\n", filename);
 #endif
 
-        if(file)
-        	*file = 0;
-        	
-	return -1;
+	if(file)
+	{
+		*file = 0;
+		return -1;
+	}
+	else
+	{
+		// When file is NULL, we're querying the existance of the file
+		// If we've got here, it doesn't exist
+		return 0;
+	}
 }
 
 /*