Moved asserts to top of EvalContextFunctionCache functions

Two reasons for this:
 1. CONTRIBUTING.md says to do this at the top
 2. the `ctx` argument was actually used before the assert

Ticket: None
Changelog: None
Signed-off-by: Lars Erik Wik <lars.erik.wik@northern.tech>

libpromises/eval_context.c

@@ -2832,16 +2832,17 @@ bool EvalContextFunctionCacheGet(const EvalContext *ctx,
                                  const FnCall *fp ARG_UNUSED,
                                  const Rlist *args, Rval *rval_out)
 {
+    assert(fp != NULL);
+    assert(fp->name != NULL);
+    assert(ctx != NULL);
+
     if (!(ctx->eval_options & EVAL_OPTION_CACHE_SYSTEM_FUNCTIONS))
     {
         return false;
     }
 
     // The cache key is made of the function name and all args values
     Rlist *args_copy = RlistCopy(args);
-    assert(fp != NULL);
-    assert(fp->name != NULL);
-    assert(ctx != NULL);
     Rlist *key = RlistPrepend(&args_copy, fp->name, RVAL_TYPE_SCALAR);
     Rval *rval = FuncCacheMapGet(ctx->function_cache, key);
     RlistDestroy(key);
@@ -2863,6 +2864,10 @@ void EvalContextFunctionCachePut(EvalContext *ctx,
                                  const FnCall *fp ARG_UNUSED,
                                  const Rlist *args, const Rval *rval)
 {
+    assert(fp != NULL);
+    assert(fp->name != NULL);
+    assert(ctx != NULL);
+
     if (!(ctx->eval_options & EVAL_OPTION_CACHE_SYSTEM_FUNCTIONS))
     {
         return;
@@ -2872,9 +2877,6 @@ void EvalContextFunctionCachePut(EvalContext *ctx,
     *rval_copy = RvalCopy(*rval);
 
     Rlist *args_copy = RlistCopy(args);
-    assert(fp != NULL);
-    assert(fp->name != NULL);
-    assert(ctx != NULL);
     Rlist *key = RlistPrepend(&args_copy, fp->name, RVAL_TYPE_SCALAR);
 
     FuncCacheMapInsert(ctx->function_cache, key, rval_copy);