cover: error on negative numbers in profiles

CL 179377 introduced an optimized parser for coverage profiles.
The parser replaces the following regex:
^(.+):([0-9]+)\.([0-9]+),([0-9]+)\.([0-9]+) ([0-9]+) ([0-9]+)$

With this regex, negative numbers in the coverage profiles resulted
in parsing errors. With the new parser in place, this is no longer
the case. This commit restores the old behavior.

Change-Id: Iaa72035f1f587ed186eaf5a84b209df88e67fb57
GitHub-Last-Rev: 07470fba9d
GitHub-Pull-Request: golang/tools#195
Reviewed-on: https://go-review.googlesource.com/c/tools/+/213357
Reviewed-by: Katharine Berry <ktbry@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

cover/profile.go

@@ -168,6 +168,9 @@ func seekBack(l string, sep byte, end int, what string) (value int, nextSep int,
 			if err != nil {
 				return 0, 0, fmt.Errorf("couldn't parse %q: %v", what, err)
 			}
+			if i < 0 {
+				return 0, 0, fmt.Errorf("negative values are not allowed for %s, found %d", what, i)
+			}
 			return i, start, nil
 		}
 	}

cover/profile_test.go

@@ -198,6 +198,12 @@ some/fancy/path:42.69,44.16 2`,
 :42.69,44.16 2 3`,
 			expectErr: true,
 		},
+		{
+			name: "a negative count is an error",
+			input: `mode: count
+some/fancy/path:42.69,44.16 2 -1`,
+			expectErr: true,
+		},
 	}
 
 	for _, tc := range tests {