lasieve4e: initialize 'ret' to avoid undefined behavior

gnfs-lasieve4e.c: In function ‘main’:
gnfs-lasieve4e.c:2987:11: warning: ‘ret’ may be used uninitialized in this function

if resume file is zero bytes, ret will be used uninitialized because
fgets will return NULL the first time it's called.  Intended behavior
is to continue without it, so start out with ret = 0.

src/lasieve4/gnfs-lasieve4e.c

@@ -2984,7 +2984,7 @@ int main(int argc, char **argv)
 
     if (g_resume != 0) {
       char buf[LINE_BUF_SIZE];
-      int ret;
+      int ret = 0;
 
       if (zip_output != 0)
 	complain("Cannot resume gzipped file. gunzip, and retry without -z\n");