Make resize! a no-op when size does not change

Else we set the last element to zero for one-byte element types like UInt8,
which means that resizing a vector allocated with StringVector corrupts it.
Also add redundant checks in Julia code to avoid a function call.

base/array.jl

@@ -936,7 +936,7 @@ function resize!(a::Vector, nl::Integer)
     l = length(a)
     if nl > l
         ccall(:jl_array_grow_end, Cvoid, (Any, UInt), a, nl-l)
-    else
+    elseif nl != l
         if nl < 0
             throw(ArgumentError("new length must be ≥ 0"))
         end

src/array.c

@@ -999,6 +999,8 @@ JL_DLLEXPORT void jl_array_del_beg(jl_array_t *a, size_t dec)
         jl_bounds_error_int((jl_value_t*)a, dec);
     if (__unlikely(a->flags.isshared))
         array_try_unshare(a);
+    if (dec == 0)
+        return;
     jl_array_del_at_beg(a, 0, dec, n);
 }
 
@@ -1009,6 +1011,8 @@ JL_DLLEXPORT void jl_array_del_end(jl_array_t *a, size_t dec)
         jl_bounds_error_int((jl_value_t*)a, 0);
     if (__unlikely(a->flags.isshared))
         array_try_unshare(a);
+    if (dec == 0)
+        return;
     jl_array_del_at_end(a, n - dec, dec, n);
 }
 

test/arrayops.jl

@@ -2342,4 +2342,19 @@ Base.view(::T25958, args...) = args
     @test t[end,1,end]   == @view(t[end,1,end])   == @views t[end,1,end]
     @test t[end,end,1]   == @view(t[end,end,1])   == @views t[end,end,1]
     @test t[end,end,end] == @view(t[end,end,end]) == @views t[end,end,end]
+end
+
+# resize! sets the last element to zero when element size is one byte
+@testset "check that resize! is a no-op when size does not change" for T in (Int, UInt8, Int8)
+    x = Vector{T}(uninitialized, 1)
+    resize!(x, 0)
+    unsafe_store!(pointer(x), 1, 1)
+    resize!(x, 0)
+    @test unsafe_load(pointer(x), 1) == 1
+
+    x = Vector{T}(uninitialized, 2)
+    resize!(x, 1)
+    unsafe_store!(pointer(x), 1, 2)
+    resize!(x, 1)
+    @test unsafe_load(pointer(x), 2) == 1
 end

test/strings/basic.jl

@@ -8,6 +8,16 @@ using Random
     @test String("abc!") == "abc!"
     @test String(0x61:0x63) == "abc"
 
+    # Check that resizing empty source vector does not corrupt string
+    b = IOBuffer()
+    write(b, "ab")
+    x = take!(b)
+    s = String(x)
+    resize!(x, 0)
+    @test s == "ab"
+    resize!(x, 1)
+    @test s == "ab"
+
     @test isempty(string())
     @test eltype(GenericString) == Char
     @test firstindex("abc") == 1