maint: avoid undefined behavior in qsort call

GCC 5.1.1 -fsanitize=undefined with glibc 2.21 is returning:
"runtime error: null pointer passed as argument 1,
 which is declared to never be null"
* src/ptx.c (sort_found_occurs): Avoid the call with no entries.

1 files changed, 3 insertions, 3 deletions

diff --git a/src/ptx.c b/src/ptx.c
index ff4e63a5f..3aea4cdf2 100644
--- a/src/ptx.c
+++ b/src/ptx.c
@@ -631,9 +631,9 @@ sort_found_occurs (void)
 {
 
   /* Only one language for the time being.  */
-
-  qsort (occurs_table[0], number_of_occurs[0], sizeof **occurs_table,
-         compare_occurs);
+  if (number_of_occurs[0])
+    qsort (occurs_table[0], number_of_occurs[0], sizeof **occurs_table,
+           compare_occurs);
 }
 
 /* Parameter files reading routines.  */