Update 3rdparty libspng to 0.7.4

Include three fixes:
https://github.com/randy408/libspng/commit/e68ba5df571726bf001d9ea150980463ea395a75
https://github.com/randy408/libspng/commit/6c7c8ce3729bb866eec9e94e426c3221abf402b0
https://github.com/randy408/libspng/commit/c9451cab664794dc909c4580dd1e6e013f3db512

and three chores:
https://github.com/randy408/libspng/commit/d86a11d3ab8f37e7772d52490d3b086d0e0f9bc7
https://github.com/randy408/libspng/commit/bab9f94a1186f8fda234b4e1cb65e1b9a77049cc
https://github.com/randy408/libspng/commit/fb768002d4288590083a476af628e51c3f1d47cd

I have reviewed CMakeLists.txt and I think disabling unused-variable
warning is not needed.

3rdparty/libspng/CMakeLists.txt

@@ -23,7 +23,6 @@ if(MSVC)
 endif(MSVC)
 
 add_library(${SPNG_LIBRARY} STATIC ${OPENCV_3RDPARTY_EXCLUDE_FROM_ALL} ${spng_headers} ${spng_sources})
-ocv_warnings_disable(CMAKE_C_FLAGS -Wunused-variable)
 target_link_libraries(${SPNG_LIBRARY} ${ZLIB_LIBRARIES})
 
 set_target_properties(${SPNG_LIBRARY}

3rdparty/libspng/LICENSE

@@ -1,6 +1,6 @@
 BSD 2-Clause License
 
-Copyright (c) 2018-2022, Randy <randy408@protonmail.com>
+Copyright (c) 2018-2023, Randy <randy408@protonmail.com>
 All rights reserved.
 
 Redistribution and use in source and binary forms, with or without

3rdparty/libspng/spng.c

@@ -2691,6 +2691,7 @@ static int read_non_idat_chunks(spng_ctx *ctx)
             if(!memcmp(chunk.type, type_exif, 4))
             {
                 if(ctx->file.exif) return SPNG_EDUP_EXIF;
+                if(!chunk.length) return SPNG_EEXIF;
 
                 ctx->file.exif = 1;
 
@@ -4999,11 +5000,11 @@ void spng_ctx_free(spng_ctx *ctx)
     spng__free(ctx, ctx->prev_scanline_buf);
     spng__free(ctx, ctx->filtered_scanline_buf);
 
-    spng_free_fn *free_func = ctx->alloc.free_fn;
+    spng_free_fn *free_fn = ctx->alloc.free_fn;
 
     memset(ctx, 0, sizeof(spng_ctx));
 
-    free_func(ctx);
+    free_fn(ctx);
 }
 
 static int buffer_read_fn(spng_ctx *ctx, void *user, void *data, size_t n)
@@ -5743,7 +5744,8 @@ int spng_set_iccp(spng_ctx *ctx, struct spng_iccp *iccp)
     SPNG_SET_CHUNK_BOILERPLATE(iccp);
 
     if(check_png_keyword(iccp->profile_name)) return SPNG_EICCP_NAME;
-    if(!iccp->profile_len || iccp->profile_len > UINT_MAX) return 1;
+    if(!iccp->profile_len) return SPNG_ECHUNK_SIZE;
+    if(iccp->profile_len > spng_u32max) return SPNG_ECHUNK_STDLEN;
 
     if(ctx->iccp.profile && !ctx->user.iccp) spng__free(ctx, ctx->iccp.profile);
 

3rdparty/libspng/spng.h

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: (BSD-2-Clause AND libpng-2.0) */
+/* SPDX-License-Identifier: BSD-2-Clause */
 #ifndef SPNG_H
 #define SPNG_H
 
@@ -28,7 +28,7 @@ extern "C" {
 
 #define SPNG_VERSION_MAJOR 0
 #define SPNG_VERSION_MINOR 7
-#define SPNG_VERSION_PATCH 3
+#define SPNG_VERSION_PATCH 4
 
 enum spng_errno
 {