avcodec/xpmdec: Move allocations down after more error checks

Fixes: Timeout
Fixes: 37035/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XPM_fuzzer-5142718576721920

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/xpmdec.c

@@ -341,9 +341,6 @@ static int xpm_decode_frame(AVCodecContext *avctx, void *data,
     if ((ret = ff_set_dimensions(avctx, width, height)) < 0)
         return ret;
 
-    if ((ret = ff_get_buffer(avctx, p, 0)) < 0)
-        return ret;
-
     if (cpp <= 0 || cpp >= 5) {
         av_log(avctx, AV_LOG_ERROR, "unsupported/invalid number of chars per pixel: %d\n", cpp);
         return AVERROR_INVALIDDATA;
@@ -360,14 +357,17 @@ static int xpm_decode_frame(AVCodecContext *avctx, void *data,
 
     size *= 4;
 
-    av_fast_padded_malloc(&x->pixels, &x->pixels_size, size);
-    if (!x->pixels)
-        return AVERROR(ENOMEM);
-
     ptr += mod_strcspn(ptr, ",") + 1;
     if (end - ptr < 1)
         return AVERROR_INVALIDDATA;
 
+    if ((ret = ff_get_buffer(avctx, p, 0)) < 0)
+        return ret;
+
+    av_fast_padded_malloc(&x->pixels, &x->pixels_size, size);
+    if (!x->pixels)
+        return AVERROR(ENOMEM);
+
     for (i = 0; i < ncolors; i++) {
         const uint8_t *index;
         int len;