go/scanner: recognize invalid floating-point constant exponent

Fixes #17621.

Change-Id: Id3e75c9b7fba2cf8e791c8817f890556ca238e9d
Reviewed-on: https://go-review.googlesource.com/32096Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/go/scanner/scanner.go

@@ -349,7 +349,11 @@ exponent:
 		if s.ch == '-' || s.ch == '+' {
 			s.next()
 		}
-		s.scanMantissa(10)
+		if digitVal(s.ch) < 10 {
+			s.scanMantissa(10)
+		} else {
+			s.error(offs, "illegal floating-point exponent")
+		}
 	}
 
 	if s.ch == 'i' {

src/go/scanner/scanner_test.go

@@ -717,6 +717,7 @@ var errors = []struct {
 	{"078.", token.FLOAT, 0, "078.", ""},
 	{"07801234567.", token.FLOAT, 0, "07801234567.", ""},
 	{"078e0", token.FLOAT, 0, "078e0", ""},
+	{"0E", token.FLOAT, 0, "0E", "illegal floating-point exponent"}, // issue 17621
 	{"078", token.INT, 0, "078", "illegal octal number"},
 	{"07800000009", token.INT, 0, "07800000009", "illegal octal number"},
 	{"0x", token.INT, 0, "0x", "illegal hexadecimal number"},