Conditionally include stdlib.h

If we are using the preallocated interface only then stdlib.h is not
required. Currently we unconditionally include it.

Add ifdef guards around any code that uses malloc and friends and then
conditionally include `stdlib.h` based on `PREALLOC_INTERFACE_ONLY`.

The benefit of this patch is that downstream users who wish to build
libsecp256k1 into WASM will not need to patch into their build an empty
`stdlib.h` file.

src/secp256k1.c

@@ -116,34 +116,42 @@ secp256k1_context* secp256k1_context_preallocated_create(void* prealloc, unsigne
 }
 
 secp256k1_context* secp256k1_context_create(unsigned int flags) {
+    secp256k1_context* ctx = NULL;
+
+#ifndef PREALLOC_INTERFACE_ONLY
     size_t const prealloc_size = secp256k1_context_preallocated_size(flags);
-    secp256k1_context* ctx = (secp256k1_context*)checked_malloc(&default_error_callback, prealloc_size);
+    ctx = (secp256k1_context*)checked_malloc(&default_error_callback, prealloc_size);
     if (EXPECT(secp256k1_context_preallocated_create(ctx, flags) == NULL, 0)) {
         free(ctx);
         return NULL;
     }
-
+#endif
     return ctx;
 }
 
 secp256k1_context* secp256k1_context_preallocated_clone(const secp256k1_context* ctx, void* prealloc) {
-    secp256k1_context* ret;
+    secp256k1_context* ret = NULL;
+
+#ifndef PREALLOC_INTERFACE_ONLY
     VERIFY_CHECK(ctx != NULL);
     ARG_CHECK(prealloc != NULL);
 
     ret = (secp256k1_context*)prealloc;
     *ret = *ctx;
+#endif
     return ret;
 }
 
 secp256k1_context* secp256k1_context_clone(const secp256k1_context* ctx) {
-    secp256k1_context* ret;
+    secp256k1_context* ret = NULL;
     size_t prealloc_size;
 
+#ifndef PREALLOC_INTERFACE_ONLY
     VERIFY_CHECK(ctx != NULL);
     prealloc_size = secp256k1_context_preallocated_clone_size(ctx);
     ret = (secp256k1_context*)checked_malloc(&ctx->error_callback, prealloc_size);
     ret = secp256k1_context_preallocated_clone(ctx, ret);
+#endif
     return ret;
 }
 

src/util.h

@@ -11,14 +11,17 @@
 #include "libsecp256k1-config.h"
 #endif
 
-#include <stdlib.h>
 #include <stdint.h>
 #include <limits.h>
 
 #if defined(VERIFY) || !defined(USE_EXTERNAL_DEFAULT_CALLBACKS)
 #include <stdio.h>
 #endif
 
+#ifndef PREALLOC_INTERFACE_ONLY
+#include <stdlib.h>
+#endif
+
 #define STR_(x) #x
 #define STR(x) STR_(x)
 #define DEBUG_CONFIG_MSG(x) "DEBUG_CONFIG: " x