noise_model.c: add missing input file check

Fixes a clang-19 static analysis warnings of the form:

```
The 1st argument to 'fopen' is NULL but should not be NULL
```

Bug: 474642915
Change-Id: I81cc75876832b112a74b6b76c8a56e0136926f49

examples/noise_model.c

diff --git a/examples/noise_model.c b/examples/noise_model.c
index e23185e..ac7f5f5 100644
--- a/examples/noise_model.c
+++ b/examples/noise_model.c
@@ -304,6 +304,9 @@
   info.time_base.numerator = args.fps.den;
   info.time_base.denominator = args.fps.num;
 
+  if (!args.input) {
+    die("No input file specified\n");
+  }
   if (info.frame_width <= 0 || info.frame_height <= 0 ||
       (info.frame_width % 2) != 0 || (info.frame_height % 2) != 0) {
     die("Invalid frame size: %dx%d", info.frame_width, info.frame_height);