Raise ArgumentError if BigFloat initialized with invalid string (crys…

…tal-lang#5638)

* Raise ArgumentError if BigFloat initialized with invalid string

Raise ArgumentError if BigFloat.new() initialized with string
that doesn't denote a valid float

* fixup! Raise ArgumentError if BigFloat initialized with invalid string

spec/std/big/big_float_spec.cr

@@ -15,6 +15,12 @@ describe "BigFloat" do
       BigFloat.new("-#{string_of_integer_value}").to_s.should eq("-#{string_of_integer_value}")
     end
 
+    it "raises an ArgumentError unless string denotes valid float" do
+      expect_raises(ArgumentError) { BigFloat.new("abc") }
+      expect_raises(ArgumentError) { BigFloat.new("+") }
+      expect_raises(ArgumentError) { BigFloat.new("") }
+    end
+
     it "new(BigInt)" do
       bigfloat_on_bigint_value = BigFloat.new(BigInt.new(string_of_integer_value))
       bigfloat_on_bigint_value.should eq(bigfloat_of_integer_value)

src/big/big_float.cr

@@ -15,7 +15,9 @@ struct BigFloat < Float
   def initialize(str : String)
     # Strip leading '+' char to smooth out cases with strings like "+123"
     str = str.lchop('+')
-    LibGMP.mpf_init_set_str(out @mpf, str, 10)
+    if LibGMP.mpf_init_set_str(out @mpf, str, 10) == -1
+      raise ArgumentError.new("Invalid BigFloat: #{str.inspect}")
+    end
   end
 
   def initialize(num : BigInt)