Fix ccall with no supplied varargs (JuliaLang/JuliaLowering.jl#102)

Just an off-by-one in desugaring's argument counting. Test case from Mmap.jl:
```
ccall(:fcntl, Cint, (RawFD, Cint, Cint...), s, F_GETFL)
```

Author: mlechu

JuliaLowering/src/desugaring.jl

@@ -1749,28 +1749,26 @@ function expand_ccall(ctx, ex)
     end
     arg_types = children(arg_type_tuple)
     vararg_type = nothing
+    num_required_args = length(arg_types)
     if length(arg_types) >= 1
         va = arg_types[end]
         if kind(va) == K"..."
             @chk numchildren(va) == 1
             # Ok: vararg function
             vararg_type = va
+            if length(arg_types) <= 1
+                throw(LoweringError(vararg_type, "C ABI prohibits vararg without one required argument"))
+            else
+                num_required_args = length(arg_types) - 1
+            end
         end
     end
     # todo: use multi-range errors here
-    if length(args) < length(arg_types)
+    if length(args) < num_required_args
         throw(LoweringError(ex, "Too few arguments in ccall compared to argument types"))
     elseif length(args) > length(arg_types) && isnothing(vararg_type)
         throw(LoweringError(ex, "More arguments than types in ccall"))
     end
-    if isnothing(vararg_type)
-        num_required_args = 0
-    else
-        num_required_args = length(arg_types) - 1
-        if num_required_args < 1
-            throw(LoweringError(vararg_type, "C ABI prohibits vararg without one required argument"))
-        end
-    end
     sctx = with_stmts(ctx)
     expanded_types = SyntaxList(ctx)
     for (i, argt) in enumerate(arg_types)
@@ -1845,7 +1843,7 @@ function expand_ccall(ctx, ex)
                     expanded_types...
                 ]
             ]
-            num_required_args::K"Integer"
+            (isnothing(vararg_type) ? 0 : num_required_args)::K"Integer"
             if isnothing(cconv)
                 "ccall"::K"Symbol"
             else

JuliaLowering/test/misc_ir.jl

@@ -396,7 +396,7 @@ end
 24  (return %₂₃)
 
 ########################################
-# @ccall lowering with varargs and gc_safe
+# @ccall lowering with gc_safe
 @ccall foo(x::X; y::Y)::R gc_safe=true
 #---------------------
 1   JuliaLowering.Base
@@ -424,6 +424,22 @@ end
 23  (foreigncall :foo (static_eval TestMod.R) (static_eval (call core.svec TestMod.X TestMod.Y)) 1 :($(QuoteNode((:ccall, 0x0000, true)))) %₁₅ %₂₀ %₂₁ %₂₂)
 24  (return %₂₃)
 
+########################################
+# non-macro ccall with vararg in signature, but none provided
+ccall(:fcntl, Cint, (RawFD, Cint, Cint...), s, F_GETFL)
+#---------------------
+1   TestMod.RawFD
+2   TestMod.Cint
+3   TestMod.Cint
+4   TestMod.s
+5   (call top.cconvert %₁ %₄)
+6   TestMod.F_GETFL
+7   (call top.cconvert %₂ %₆)
+8   (call top.unsafe_convert %₁ %₅)
+9   (call top.unsafe_convert %₂ %₇)
+10  (foreigncall :fcntl (static_eval TestMod.Cint) (static_eval (call core.svec TestMod.RawFD TestMod.Cint TestMod.Cint)) 2 :ccall %₈ %₉ %₅ %₇)
+11  (return %₁₀)
+
 ########################################
 # Error: No return annotation on @ccall
 @ccall strlen("foo"::Cstring)