ffprobe: rework/fix flat writer

Do not build from scratch the section header for each section, but build
using the previous level buffer, thus improving efficiency and fix some
few corner cases which are exposed by the pending disposition patch.

ffprobe.c

@@ -898,35 +898,32 @@ static void flat_print_section_header(WriterContext *wctx)
 {
     FlatContext *flat = wctx->priv;
     AVBPrint *buf = &flat->section_header[wctx->level];
-    int i;
+    const struct section *section = wctx->section[wctx->level];
+    const struct section *parent_section = wctx->level ?
+        wctx->section[wctx->level-1] : NULL;
 
     /* build section header */
     av_bprint_clear(buf);
-    for (i = 1; i <= wctx->level; i++) {
-        if (flat->hierarchical ||
-            !(wctx->section[i]->flags & (SECTION_FLAG_IS_ARRAY|SECTION_FLAG_IS_WRAPPER)))
-            av_bprintf(buf, "%s%s", wctx->section[i]->name, flat->sep_str);
-    }
-}
-
-static void flat_print_key_prefix(WriterContext *wctx)
-{
-    FlatContext *flat = wctx->priv;
-    const struct section *parent_section = wctx->section[wctx->level-1];
+    if (!parent_section)
+        return;
+    av_bprintf(buf, "%s", flat->section_header[wctx->level-1].str);
 
-    printf("%s", flat->section_header[wctx->level].str);
+    if (flat->hierarchical ||
+        !(section->flags & (SECTION_FLAG_IS_ARRAY|SECTION_FLAG_IS_WRAPPER))) {
+        av_bprintf(buf, "%s%s", wctx->section[wctx->level]->name, flat->sep_str);
 
         if (parent_section->flags & SECTION_FLAG_IS_ARRAY) {
             int n = parent_section->id == SECTION_ID_PACKETS_AND_FRAMES ?
                 wctx->nb_section_packet_frame : wctx->nb_item[wctx->level-1];
-        printf("%d%s", n, flat->sep_str);
+            av_bprintf(buf, "%d%s", n, flat->sep_str);
+        }
     }
 }
 
 static void flat_print_int(WriterContext *wctx, const char *key, long long int value)
 {
-    flat_print_key_prefix(wctx);
-    printf("%s=%lld\n", key, value);
+    FlatContext *flat = wctx->priv;
+    printf("%s%s=%lld\n", flat->section_header[wctx->level].str, key, value);
 }
 
 static void flat_print_str(WriterContext *wctx, const char *key, const char *value)
@@ -934,7 +931,7 @@ static void flat_print_str(WriterContext *wctx, const char *key, const char *val
     FlatContext *flat = wctx->priv;
     AVBPrint buf;
 
-    flat_print_key_prefix(wctx);
+    printf("%s", flat->section_header[wctx->level].str);
     av_bprint_init(&buf, 1, AV_BPRINT_SIZE_UNLIMITED);
     printf("%s=", flat_escape_key_str(&buf, key, flat->sep));
     av_bprint_clear(&buf);