bufferobj: init the return value for GetParam functions

It's helping buggy applications that may use the returned
value without checking for an error first.

For instance, viewperf20/maya uses a sequence like this:
   glGenBuffers(n = 1, buffers = &3458)
   ...
   glGetNamedBufferParameteriv(3458, GL_BUFFER_SIZE, &p)

Since 3458 isn't associated to an object yet, _mesa_lookup_bufferobj_err
will return NULL, leaving 'p' with its original value.
It seems to randomly trigger an exit from the benchmark,
presumably because the content in 'p' is random (for instance
-1879044180).

Cc: mesa-stable

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/36190>

src/mesa/main/bufferobj.c

@@ -3162,6 +3162,8 @@ _mesa_GetBufferParameteriv(GLenum target, GLenum pname, GLint *params)
    struct gl_buffer_object *bufObj;
    GLint64 parameter;
 
+   *params = 0;
+
    bufObj = get_buffer(ctx, "glGetBufferParameteriv", target,
                        GL_INVALID_OPERATION);
    if (!bufObj)
@@ -3181,6 +3183,8 @@ _mesa_GetBufferParameteri64v(GLenum target, GLenum pname, GLint64 *params)
    struct gl_buffer_object *bufObj;
    GLint64 parameter;
 
+   *params = 0;
+
    bufObj = get_buffer(ctx, "glGetBufferParameteri64v", target,
                        GL_INVALID_OPERATION);
    if (!bufObj)
@@ -3200,6 +3204,8 @@ _mesa_GetNamedBufferParameteriv(GLuint buffer, GLenum pname, GLint *params)
    struct gl_buffer_object *bufObj;
    GLint64 parameter;
 
+   *params = 0;
+
    bufObj = _mesa_lookup_bufferobj_err(ctx, buffer,
                                        "glGetNamedBufferParameteriv");
    if (!bufObj)
@@ -3219,6 +3225,8 @@ _mesa_GetNamedBufferParameterivEXT(GLuint buffer, GLenum pname, GLint *params)
    struct gl_buffer_object *bufObj;
    GLint64 parameter;
 
+   *params = 0;
+
    if (!buffer) {
       _mesa_error(ctx, GL_INVALID_OPERATION,
                   "glGetNamedBufferParameterivEXT: buffer=0");
@@ -3245,6 +3253,8 @@ _mesa_GetNamedBufferParameteri64v(GLuint buffer, GLenum pname,
    struct gl_buffer_object *bufObj;
    GLint64 parameter;
 
+   *params = 0;
+
    bufObj = _mesa_lookup_bufferobj_err(ctx, buffer,
                                        "glGetNamedBufferParameteri64v");
    if (!bufObj)