charsets: Optimize ENC_ASCII by appending valid bytes at once

For ASCII encoding, most bytes are copied directly. Count consecutive
valid bytes in an accumulator and append them all at once when we
get an invalid character with the high bit set, or at the end.
This reduces the number of reallocations and allows larger, more
optimized memcpys.

epan/charsets.c

@@ -72,19 +72,29 @@ guint8 *
 get_ascii_string(wmem_allocator_t *scope, const guint8 *ptr, gint length)
 {
     wmem_strbuf_t *str;
+    const guint8 *prev = ptr;
+    size_t valid_bytes = 0;
 
     str = wmem_strbuf_new_sized(scope, length+1);
 
     while (length > 0) {
-        guint8 ch = *ptr;
+        guint8 ch = *ptr++;
 
-        if (ch < 0x80)
-            wmem_strbuf_append_c(str, ch);
-        else
+        if (ch < 0x80) {
+            valid_bytes++;
+        } else {
+            if (valid_bytes) {
+                wmem_strbuf_append_len(str, prev, valid_bytes);
+                prev = ptr;
+                valid_bytes = 0;
+            }
             wmem_strbuf_append_unichar_repl(str);
-        ptr++;
+        }
         length--;
     }
+    if (valid_bytes) {
+        wmem_strbuf_append_len(str, prev, valid_bytes);
+    }
 
     return (guint8 *) wmem_strbuf_finalize(str);
 }