check input value to enforce pre-condition

Author: frederic-mahe

src/vsearch.cc

@@ -5771,6 +5771,9 @@ auto cmd_fastq_mergepairs() -> void
     {
       fatal("No output files specified");
     }
+    if (opt_fastq_maxdiffs < 0) {
+      fatal("Argument to --fastq_maxdiffs must be positive");
+    }
   fastq_mergepairs();
 }