Do not pass NULL to memset in ssa_global_cache.

The code computing ranges in PHIs in the path solver reuses the
temporary ssa_global_cache by calling its clear method.  Calling it on
an empty cache causes us to call memset with NULL.

Tested on x86-64 Linux.

gcc/ChangeLog:

	PR tree-optimization/103229
	* gimple-range-cache.cc (ssa_global_cache::clear): Do not pass
	null value to memset.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr103229.c: New test.

gcc/gimple-range-cache.cc

@@ -651,7 +651,8 @@ ssa_global_cache::clear_global_range (tree name)
 void
 ssa_global_cache::clear ()
 {
-  memset (m_tab.address(), 0, m_tab.length () * sizeof (irange *));
+  if (m_tab.address ())
+    memset (m_tab.address(), 0, m_tab.length () * sizeof (irange *));
 }
 
 // Dump the contents of the global cache to F.

gcc/testsuite/gcc.dg/pr103229.c

@@ -0,0 +1,10 @@
+// { dg-do compile }
+// { dg-options "-O -w" }
+
+int main() {
+  int i;
+  for (; i;)
+    ;
+
+  return 0;
+}