Add a fast path for returning "" from repeat(str, 0) (#35579)

Currently the case where `r == 0` falls through the same logic as every
other non-negative value of `r` (aside from 1). This works for signed
integers. However, this does not work for unsigned integers: in the loop
where we unsafely fill in the output string, we're looping from 0 to `r
- 1`, which for unsigned integers wraps around and causes us to request
the address of the output string at a location that is well beyond what
was allocated.

Fixes #35578.

base/strings/substring.jl

@@ -198,6 +198,7 @@ end
 
 function repeat(s::Union{String, SubString{String}}, r::Integer)
     r < 0 && throw(ArgumentError("can't repeat a string $r times"))
+    r == 0 && return ""
     r == 1 && return String(s)
     n = sizeof(s)
     out = _string_n(n*r)

test/strings/basic.jl

@@ -668,15 +668,17 @@ end
         @test_throws ArgumentError repeat(c, -1)
         @test_throws ArgumentError repeat(s, -1)
         @test_throws ArgumentError repeat(S, -1)
-        @test repeat(c, 0) === ""
-        @test repeat(s, 0) === ""
-        @test repeat(S, 0) === ""
-        @test repeat(c, 1) === s
-        @test repeat(s, 1) === s
-        @test repeat(S, 1) === S
-        @test repeat(c, 3) === S
-        @test repeat(s, 3) === S
-        @test repeat(S, 3) === S*S*S
+        for T in (Int, UInt)
+            @test repeat(c, T(0)) === ""
+            @test repeat(s, T(0)) === ""
+            @test repeat(S, T(0)) === ""
+            @test repeat(c, T(1)) === s
+            @test repeat(s, T(1)) === s
+            @test repeat(S, T(1)) === S
+            @test repeat(c, T(3)) === S
+            @test repeat(s, T(3)) === S
+            @test repeat(S, T(3)) === S*S*S
+        end
     end
     # Issue #32160 (string allocation unsigned overflow)
     @test_throws OutOfMemoryError repeat('x', typemax(Csize_t))