Make the segfault great again.

When copying the final part of the string, after the last found index,
I forgot to subtract the length of the word of the replacement.
It would copy too much bytes this way.

memcpy(temp, pstr, filelen - j) --> segfault
memcpy(temp, pstr, filelen - (j + oldLen)) --> :)

fastreplacestring.cpp

@@ -133,7 +133,7 @@ int main(int argc, char **argv) {
       temp += newLen;
       start = j + oldLen;
     }
-    memcpy(temp, pstr, filelen - j);
+    memcpy(temp, pstr, filelen - start);
 
     free(s);