avoid undefined behavior when parsing proc maps

Clang's undefined behavior sanitized correctly pointed out that
memmove requires non-nil args, so lets initialize sbuf to something.

src/base/proc_maps_iterator.cc

@@ -258,8 +258,10 @@ bool ForEachLine(const char* path, const Body& body) {
     return false;
   }
 
-  char* sbuf = nullptr;
-  char* ebuf = nullptr;
+  char* sbuf = buf; // note, initial value could be nullptr, but
+                    // memmove actually insists to get non-null
+                    // arguments (even when count is 0)
+  char* ebuf = sbuf;
 
   bool eof = false;