PR26453 UBSAN: som.c:2885 null pointer memcpy

PR 26453
	* som.c (som_prep_for_fixups): Return early when no symbols.

bfd/ChangeLog

@@ -1,3 +1,8 @@
+2020-08-26  Alan Modra  <amodra@gmail.com>
+
+	PR 26453
+	* som.c (som_prep_for_fixups): Return early when no symbols.
+
 2020-08-26  Alan Modra  <amodra@gmail.com>
 
 	PR 26418

bfd/som.c

@@ -2804,6 +2804,9 @@ som_prep_for_fixups (bfd *abfd, asymbol **syms, unsigned long num_syms)
   asymbol **sorted_syms;
   size_t amt;
 
+  if (num_syms == 0)
+    return TRUE;
+
   /* Most SOM relocations involving a symbol have a length which is
      dependent on the index of the symbol.  So symbols which are
      used often in relocations should have a small index.  */