Fix uninitialized variable use (reported by Clang Static Analyzer)

main.c:488:22: warning: The left operand of '/' is a garbage value
    fbsize = (fbsize / BLOCKSIZE) * BLOCKSIZE;
              ~~~~~~ ^
1 warning generated.

main.c

@@ -483,7 +483,7 @@ int main(void)
     void *poolbuf;
     size_t bufsize = SIZE;
 #ifdef __linux__
-    size_t fbsize;
+    size_t fbsize = 0;
     int64_t *fbbuf = mmap_framebuffer(&fbsize);
     fbsize = (fbsize / BLOCKSIZE) * BLOCKSIZE;
 #endif