Fix Math::BigFloat::sqrt() breaking with too many digits. (Closes: #417528)

Dual-lived module, fixed on the CPAN side in 1.89.

Integrated with the other 1.89 changes in blead as change 33715
and maint-5.10 as change 33821.

[rt.cpan.org #34459]
diff --git a/lib/Math/BigFloat.pm b/lib/Math/BigFloat.pm
index 6e1ecc8..1c1fba8 100644
--- a/lib/Math/BigFloat.pm
+++ b/lib/Math/BigFloat.pm
@@ -2142,8 +2142,9 @@ sub bsqrt
   # But we need at least $scale digits, so calculate how many are missing
   my $shift = $scale - $digits;
 
-  # That should never happen (we take care of integer guesses above)
-  # $shift = 0 if $shift < 0; 
+  # This happens if the input had enough digits 
+  # (we take care of integer guesses above)
+  $shift = 0 if $shift < 0; 
 
   # Multiply in steps of 100, by shifting left two times the "missing" digits
   my $s2 = $shift * 2;
diff --git a/lib/Math/BigInt/t/mbimbf.t b/lib/Math/BigInt/t/mbimbf.t
index fae3c8c..88201e1 100644
--- a/lib/Math/BigInt/t/mbimbf.t
+++ b/lib/Math/BigInt/t/mbimbf.t
@@ -32,7 +32,7 @@ BEGIN
   print "# INC = @INC\n";
 
   plan tests => 684 
-    + 23;		# own tests
+    + 26;		# own tests
   }
 
 use Math::BigInt 1.70;
@@ -100,3 +100,9 @@ $x = Math::BigFloat->new(100);
 $x = $x->blog(Math::BigInt->new(10));
 
 ok ($x,2);
+
+for my $i (80,88,100) {
+    $x = Math::BigFloat->new("1." . ("0" x $i) . "1");
+    $x = $x->bsqrt;
+    ok ($x, 1);
+}