common: restore OpenMP thread limit after GraphicsMagick init

GraphicsMagick resets the main thread's OpenMP limit to the CPU count.
With --threads set lower, darktable-cli can then run more threads than
its per-thread buffers hold. Reapply the requested limit after Magick
initialization.

Fixes #22436

Author: kofa73

RELEASE_NOTES.md

@@ -513,6 +513,9 @@ changes (where available).
   one, which slightly changed the colors along the right edge of the
   image.
 
+- Fixed darktable-cli crashing when `--threads` was set below the
+  number of CPUs.
+
 ## Lua
 
 ### API Version

src/common/darktable.c

@@ -2062,6 +2062,13 @@ int dt_init(int argc,
   MagickWandGenesis();
 #endif
 
+#ifdef _OPENMP
+  // GraphicsMagick's init resets the thread count to the number of CPUs,
+  // overriding --threads. dt_alloc_perthread() pools are sized by
+  // dt_get_num_threads(), so a larger team would write past their end.
+  omp_set_num_threads(darktable.num_openmp_threads);
+#endif
+
 #ifdef HAVE_LIBHEIF
   dt_splash_screen_set_progress(_("initializing libheif"));
   heif_init(NULL);