Fix escaped '%' check when building Printf.Format

Addresses
JuliaLang#32859 (comment).
The issue here was advancing the parsing position too far when an
escaped `'%'` was encountered after the first format specifier. If an
escaped `'%'` was then followed by another specifier, it was ignored due
to being classified as an "escaped" character.

stdlib/Printf/src/Printf.jl

@@ -165,15 +165,13 @@ function Format(f::AbstractString)
  end
  push!(fmts, Spec{type}(leftalign, plus, space, zero, hash, width, precision))
  start = pos
- prevperc = false
  while pos <= len
  b = bytes[pos]
  pos += 1
  if b == UInt8('%')
  pos > len && throw(ArgumentError("invalid format string: '$f'"))
  if bytes[pos] == UInt8('%')
- pos += 1
- pos > len && break
+ # escaped '%'
  b = bytes[pos]
  pos += 1
  else

stdlib/Printf/test/runtests.jl

@@ -414,6 +414,11 @@ end
  @test Printf.@sprintf("%f", 1) == "1.000000"
  @test Printf.@sprintf("%e", 1) == "1.000000e+00"
  @test Printf.@sprintf("%g", 1) == "1"
+
+ # escaped '%'
+ @test_throws ArgumentError @sprintf("%s%%%s", "a")
+ @test @sprintf("%s%%%s", "a", "b") == "a%%b"
+
 end
 
 @testset "integers" begin