Fix: Bug reported by the clang static analyzer.

This issue was hard to fix, and still not sure if it is the right solution.
Please do a deep review and try running the scan analyzer again without this patch. See the report check if it is the right solution,
or I am introducing a new bug.

Description: Assigned value is garbage or undefined
File: /home/jnicola/dev/openvas/nasl/charcnv.c
Line: 625

nasl/charcnv.c

@@ -91,7 +91,7 @@ check_dos_char_slowly_ntlmssp (uint16 c)
 
   len1 = convert_string_ntlmssp (CH_UTF16LE, CH_DOS, &c, 2, buf, sizeof (buf),
                                  False);
-  if (len1 == 0)
+  if (len1 <= 0)
     {
       return 0;
     }