Throw ArgumentError if unsafe_SecretBuffer!() is passed NULL

Previously, if given a NULL `Cstring` we would blithely call `strlen()`
on it, which resulted in a segfault.  It is better if we throw an
exception instead.

base/secretbuffer.jl

@@ -79,7 +79,13 @@ function SecretBuffer!(d::Vector{UInt8})
     s
 end
 
-unsafe_SecretBuffer!(s::Cstring) = unsafe_SecretBuffer!(convert(Ptr{UInt8}, s), Int(ccall(:strlen, Csize_t, (Cstring,), s)))
+function unsafe_SecretBuffer!(s::Cstring)
+    if s == C_NULL
+        throw(ArgumentError("cannot convert NULL to SecretBuffer"))
+    end
+    len = Int(ccall(:strlen, Csize_t, (Cstring,), s))
+    unsafe_SecretBuffer!(convert(Ptr{UInt8}, s), len)
+end
 function unsafe_SecretBuffer!(p::Ptr{UInt8}, len=1)
     s = SecretBuffer(sizehint=len)
     for i in 1:len

test/secretbuffer.jl

@@ -122,4 +122,8 @@ using Test
         @test hash(sb1, UInt(5)) === hash(sb2, UInt(5))
         shred!(sb1); shred!(sb2)
     end
+    @testset "NULL initialization" begin
+        null_ptr = Cstring(C_NULL)
+        @test_throws ArgumentError Base.unsafe_SecretBuffer!(null_ptr)
+    end
 end