llext: check for an allocation failure

Add a missing allocation failure check in llext_export_symbols().

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
diff --git a/subsys/llext/llext.c b/subsys/llext/llext.c
index 02ed0e1..6c85a63 100644
--- a/subsys/llext/llext.c
+++ b/subsys/llext/llext.c
@@ -376,6 +376,9 @@
 	size_t syms_size = sym_tab->sym_cnt * sizeof(struct llext_symbol);
 
 	sym_tab->syms = k_heap_alloc(&llext_heap, syms_size, K_NO_WAIT);
+	if (!sym_tab->syms) {
+		return -ENOMEM;
+	}
 	memset(sym_tab->syms, 0, syms_size);
 	ext->mem_size += syms_size;