Do not call memcpy with a NULL argument (PR gcov-profile/80413).

2017-04-13  Martin Liska  <mliska@suse.cz>

	PR gcov-profile/80413
	* gcov-io.c (gcov_write_string): Copy to buffer just when
	allocated size is greater than zero.

From-SVN: r246903

gcc/ChangeLog

+2017-04-13  Martin Liska  <mliska@suse.cz>
+
+	PR gcov-profile/80413
+	* gcov-io.c (gcov_write_string): Copy to buffer just when
+	allocated size is greater than zero.
+
 2017-04-13  Jakub Jelinek  <jakub@redhat.com>
 
 	PR debug/80321

gcc/gcov-io.c

@@ -347,8 +347,12 @@ gcov_write_string (const char *string)
   buffer = gcov_write_words (1 + alloc);
 
   buffer[0] = alloc;
-  buffer[alloc] = 0;
-  memcpy (&buffer[1], string, length);
+
+  if (alloc > 0)
+    {
+      buffer[alloc] = 0; /* place nul terminators.  */
+      memcpy (&buffer[1], string, length);
+    }
 }
 #endif