tiny_psnr: check for specified sample size less than 1

A zero or negative sample size is impossible and should be
reported as an error.

Signed-off-by: Mans Rullgard <mans@mansr.com>

tests/tiny_psnr.c

@@ -139,7 +139,7 @@ int main(int argc, char *argv[])
         } else {
             char *end;
             len = strtol(argv[3], &end, 0);
-            if (*end || len > 2) {
+            if (*end || len < 1 || len > 2) {
                 fprintf(stderr, "Unsupported sample format: %s\n", argv[3]);
                 return 1;
             }