[WebAssembly] Fix validation of start function

[sbc100]

Summary

The check for valid start function was inverted. Added a new
test in test/Object to check this case and fixed the existing
tests in for ObjectYAML.

[sbc100]

fix condition

[dschuff]

I guess lib/object isn't really the place to be validating things like the signature of the start function...

lib/Object/WasmObjectFile.cpp

Error WasmObjectFile::parseExportSection(const uint8_t *Ptr, const uint8_t *End) {
   if (Ptr != End)
     return make_error<GenericBinaryError>("Export section ended prematurely",
                                           object_error::parse_failed);
   return Error::success();
 }
 
 Error WasmObjectFile::parseStartSection(const uint8_t *Ptr, const uint8_t *End) {
   StartFunction = readVaruint32(Ptr);
-  if (StartFunction < FunctionTypes.size())
+  if (StartFunction >= FunctionTypes.size())
     return make_error<GenericBinaryError>("Invalid start function",
                                           object_error::parse_failed);
   return Error::success();
 }
 
 Error WasmObjectFile::parseCodeSection(const uint8_t *Ptr, const uint8_t *End) {
   uint32_t FunctionCount = readVaruint32(Ptr);
   if (FunctionCount != FunctionTypes.size()) {

test/Object/wasm-invalid-start.test

+# RUN: yaml2obj %s | not llvm-objdump -h - 2>&1 | FileCheck %s
+
+!WASM
+FileHeader:
+  Version:         0x00000001
+Sections:
+  - Type:            START
+    StartFunction:   0
+
+# CHECK: {{.*}}: Invalid start function

test/ObjectYAML/wasm/function_section.yaml

 # RUN: yaml2obj %s | obj2yaml | FileCheck %s
 --- !WASM
 FileHeader:
   Version:         0x00000001
 Sections:
   - Type:            FUNCTION
-    FunctionTypes:   
-      - 1
-      - 0
+    FunctionTypes: [ 1, 0 ]
 ...
 # CHECK: --- !WASM
 # CHECK: FileHeader:
 # CHECK:   Version:           0x00000001
 # CHECK: Sections:
 # CHECK:   - Type:            FUNCTION
 # CHECK:     FunctionTypes: [ 1, 0 ]
 # CHECK: ...

test/ObjectYAML/wasm/start_section.yaml

 # RUN: yaml2obj %s | obj2yaml | FileCheck %s
+
 --- !WASM
 FileHeader:
   Version:         0x00000001
 Sections:
+  - Type:            TYPE
+    Signatures:
+      - ReturnType:      I32
+        ParamTypes:
+          - F32
+          - F32
+  - Type:            FUNCTION
+    FunctionTypes: [ 0, 0, 0 ]
   - Type:            START
     StartFunction:   1
 ...
 # CHECK: --- !WASM
 # CHECK: FileHeader:
 # CHECK:   Version:           0x00000001
 # CHECK: Sections:
 # CHECK:  - Type:            START
 # CHECK:    StartFunction:   1
 # CHECK: ...