read.c: fix empty struct initializers (#2661)

Follow up to:
640df130 Use C99 syntax to initialize avifFileType struct (#2657)

with `-DAVIF_ENABLE_EXPERIMENTAL_MINI=1`.

Empty initializers are a GNU/C23 extension. Use `{ 0 }` for C99
compatibility.

src/read.c

diff --git a/src/read.c b/src/read.c
index 841cbbf..85ae775 100644
--- a/src/read.c
+++ b/src/read.c
@@ -4060,7 +4060,7 @@
     uint8_t tmapMatrixCoefficients = AVIF_MATRIX_COEFFICIENTS_IDENTITY;
     uint32_t tmapFullRange = AVIF_FALSE;
     uint32_t hasClli = AVIF_FALSE, tmapHasClli = AVIF_FALSE;
-    avifContentLightLevelInformationBox clli = {}, tmapClli = {};
+    avifContentLightLevelInformationBox clli = { 0 }, tmapClli = { 0 };
     if (hasHdr) {
         AVIF_CHECKERR(avifROStreamReadBitsU32(&s, &hasGainmap, 1), AVIF_RESULT_BMFF_PARSE_FAILED); // bit(1) gainmap_flag;
         if (hasGainmap) {