Combine adjacent mappings even if offsets are unavailable

Some profile handlers will split mappings into multiple adjacent
entries. This interferes with pprof when overriding the main binary.

Currently there is code that attempts to combine these mappings, but
it fails if the mapping offsets aren't available. Add a check to
combine them in that situation.

profile/profile.go

 		mappings := []*Mapping{p.Mapping[0]}
 		for _, m := range p.Mapping[1:] {
 			lm := mappings[len(mappings)-1]
-			if offset := lm.Offset + (lm.Limit - lm.Start); lm.Limit == m.Start &&
-				offset == m.Offset &&
-				(lm.File == m.File || lm.File == "") {
-				lm.File = m.File
+			if adjacent(lm, m) {
 				lm.Limit = m.Limit
-				if lm.BuildID == "" {
+				if m.File != "" {
+					lm.File = m.File
+				}
+				if m.BuildID != "" {
 					lm.BuildID = m.BuildID
 				}
 				p.updateLocationMapping(m, lm)
 	}
 }
 
+// adjacent returns whether two mapping entries represent the same
+// mapping that has been split into two. Check that their addresses are adjacent,
+// and if the offsets match, if they are available.
+func adjacent(m1, m2 *Mapping) bool {
+	if m1.File != "" && m2.File != "" {
+		if m1.File != m2.File {
+			return false
+		}
+	}
+	if m1.BuildID != "" && m2.BuildID != "" {
+		if m1.BuildID != m2.BuildID {
+			return false
+		}
+	}
+	if m1.Limit != m2.Start {
+		return false
+	}
+	if m1.Offset != 0 && m2.Offset != 0 {
+		offset := m1.Offset + (m1.Limit - m1.Start)
+		if offset != m2.Offset {
+			return false
+		}
+	}
+	return true
+}
+
 func (p *Profile) updateLocationMapping(from, to *Mapping) {
 	for _, l := range p.Location {
 		if l.Mapping == from {